Caramelized Onion, Whole Grain Mustard, and Gruyere Grilled Cheese

Today is National Grilled Cheese Day. What a glorious holiday, eh? I picture National Grilled Cheese Day as a true national holiday. Marching bands playing down Main Street. Girl Scouts holding giant felt banners that say “HAVE A GOUDA DAY.” People going around with cheese stick walrus tusks in their mouth.

Moms and Dads having a homemade grilled cheese contest to see who can get the most perfect golden crisp. Dairy-free people eating soy grilled cheese and crying because they’re sad they can’t have the real stuff. And people embracing them anyways because the cheese love is so strong.

But if I just may, I’m going to suggest something crazy. The star of the show in this Gruyere grilled cheese? Well, it’s not the cheese. It’s the mustard. Specifically, old style whole grain mustard from Maille. It takes this Gruyere grilled cheese sandwich from ordinary to extraordinary. With the incorporation of Maille, this grilled cheese becomes the epitome of quality and simplicity. It pairs perfectly with the soft gruyere, and cuts the sweet caramelized onions just perfectly with its slight acidity.

I’m going to let you in on a little secret: it’s pretty darn special.

Caramelized Onion, Whole Grain Mustard, and Gruyere Grilled Cheese sandwiches

What’s the best bread for grilled cheese?

In my humble opinion, a fresh loaf of sourdough is the only bread you should be using to make homemade grilled cheese sandwiches. Sourdough has a slight tanginess to it that pairs beautifully with any cheese. Plus, sourdough is incredibly sturdy and doesn’t toast too fast in the pan. I like the way sourdough gets slightly crispy crunchy on the outside when used for grilled cheese sandwiches, and how the inside of the bread still stays relatively fluffy. 

Oh, hold on a minute. I got a little drool on my keyboard. 

stack of Caramelized Onion, Whole Grain Mustard, and Gruyere Grilled Cheese sandwiches

How to make the perfect grilled cheese

Believe it or not, making the perfect grilled cheese sandwich is a fine art form. Too often, people overcook the bread, or they use the wrong kind of cheese, or something equally disastrous. Here’s how I make the ultimate Gruyere grilled cheese every time: 

Use mayo instead of butter — I know this seems strange, but using mayo on the outside of the bread instead of butter is a total game changer. Mayo is easier to spread, so you actually need to use less of it. Because of this, the sourdough is able to crisp up without feeling greasy. 

Use grated cheese — Freshly grated cheese melts better than the sliced stuff. Granted, it’s a little trickier to pile the cheese on top of the sandwich this way, but in the end it’s totally worth it. 

Add a little mustard — Whether you’re making this Gruyere grilled cheese or another grilled cheese, a bit of mustard on your sandwich is crucial for cutting through the rich cheese and crispy bread. 

Keep the skillet over medium heat — Making a Gruyere grilled cheese is a bit of a Goldilocks moment. If the heat is too low, the bread won’t crisp up on the outside; but if the heat is too high the bread will burn and the cheese won’t have time to melt. Keep the temperature at medium heat for the best results. 

Caramelized Onion, Whole Grain Mustard, and Gruyere Grilled Cheese

With National Grilled Cheese Day here, it’s time to treat yourself to something extraordinary. Say cheese!


Gruyere Grilled Cheese with Carmelized Onions and Mustard

A Caramelized Onion, Whole Grain Mustard, and Gruyere Grilled Cheese perfect for spring. Just look at that gorgeous cheese pull!!!

A Caramelized Onion, Whole Grain Mustard, and Gruyere Grilled Cheese perfect for spring. Just look at that gorgeous cheese pull!

  • Author: Sarah | Broma Bakery


For the caramelized onions (will make enough for 3 grilled cheeses):

  • 4 large white or yellow onions, halved and sliced into strips
  • 1/4 cup plus 1 tablespoon olive oil
  • 1/4 cup water
  • 1/4 teaspoon salt

For the grilled cheese:

  • 2 slices fresh sourdough bread
  • 1 tablespoon mayonnaise
  • 2 oz gruyere cheese, grated
  • 2 tablespoons Maille Old Style Whole Grain Dijon Mustard
  • 3 tablespoons caramelized onions, from above
  • a few sprigs of fresh thyme


  1. Heat a heavy bottomed saucepan over medium heat. Add in onions and 1 tablespoon olive oil. Cook for 1 minute until heated through, then turn heat down to low and add in 1/4 cup water, remaining 1/4 cup olive oil, and salt.
  2. Cook, stirring occasionally, for 1 hour. The onions will go from white to translucent, and finally will begin to turn golden brown. If the onions are looking dry, add 1-2 additional tablespoons of water, as necessary.
  3. Once onions have reached a nice medium golden brown color, remove from heat and allow to cool slightly. Onions can be stored for up to 1 week in the fridge.
  4. To make the grilled cheese, heat a skillet over medium heat. Spread mayonnaise over both slices of sourdough, then spread Maille Old Style Whole Grain Dijon Mustard over the opposite side of one of the slices. Place both slices, mayo side down, onto skillet.
  5. Dollop the mustard slice of bread with caramelized onions and a sprinkle on few leaves of fresh thyme. Carefully place grated gruyere over the other slice. Cook for 2-3 minutes, until cheese is melted completely.
  6. Press sandwich together, cut on the diagonal, and serve!

More easy lunch recipes from Broma Bakery: 

Green Falafel Sandwich with Beet Hummus

Slow Cooker Asian BBQ Pulled Chicken Sandwiches

Spaghetti with White Wine & Roasted Tomato Cream Sauce

My Take On Ottolenghi’s Fattoush Salad

Lightened Up Creamy Chicken Corn Chowder

Caramelized Onion, Whole Grain Mustard, and Gruyere Grilled Cheese

This post is sponsored by Maille. Thank you for continuing to support the brands that help make Broma possible!



Mustard, caramelised onions and gruyère? YUM! Count my boyfriend and I in! Pinned for this weekend 😉


National Grilled Cheese Day? Hooray! And thank you! ????❤️

Jennifer Farley

I am absolutely making this for lunch this weekend!


Love this dijon in this!

Leigh Ann

What a dreamy combo! That gooey cheese is everything!

Natasha @ Salt & Lavender

Love Maille Old Style mustard. I use it in a lot of my recipes. I also love your description of the national holiday hahaha.

Matt Robinson

You took grilled cheese to a whole new level with this!

Laura | Tutti Dolci

You had me at caramelized onions, these flavors are perfection!

Mary Ann | The Beach House Kitchen

Great way to amp up the classic grilled cheese!


wow, I’m breaking my diet for this, not too bad on the calorie count but love these flavors, thank you for this inspired grill cheese, the adult version!

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