Caramelized Onion Grilled Cheese
This caramelized onion grilled cheese is the grown-up grilled cheese of my dreams. Jammy, balsamic caramelized onions get tucked between sharp cheddar and nutty Gruyère on thick-cut sourdough, with a swipe of Dijon inside and mayo outside for the crispiest, most golden crust. This gourmet grilled cheese is a little bit French onion soup and a whole lot of cozy.
What better way to ring in fall than with the best pairing ever: grilled cheese and tomato soup. Serve it with my Roasted Tomato Basil Soup and thank me later.
Why You’ll Love This Caramelized Onion Grilled Cheese
- Sweet, jammy onions that taste like French onion soup in sandwich form.
- Two cheeses for the perfect melt and flavor.
- Mayo on the outside for an extra crispy, golden crust.
- Cozy and impressive but made with simple ingredients.
Ingredients for Caramelized Onion Grilled Cheese
The Caramelized Onions
Yellow onions, butter, olive oil, salt, black pepper and a splash of balsamic vinegar for depth.
The Cheese
Shredded sharp cheddar and Gruyère. Shredding your own melts much better than pre-shredded.
The Bread and Spreads
Thick-cut sourdough (or your favorite bread), Dijon mustard for the inside and mayo for the outside.
For the Pan
A little avocado oil and butter.
Swap note: Use your favorite gluten-free bread to make this gluten-free. Swiss or provolone can stand in for the Gruyère if that is what you have.
How to Make Caramelized Onion Grilled Cheese
Step 1: Start the onions
Heat the butter and olive oil in a skillet over medium-low heat. Add the sliced onions and season with salt and pepper.
Step 2: Caramelize low and slow
Cook 40 to 50 minutes, stirring often, until the onions are deeply golden and jammy. Stir in the balsamic vinegar during the last 10 minutes.
Step 3: Prep the bread
Spread mayo on one side of each slice of bread. Flip the slices over and spread Dijon on the other side.
Step 4: Build the sandwiches
Divide the cheddar, Gruyère and caramelized onions between two slices, Dijon side up. Top with the remaining bread, mayo side out.
Step 5: Grill
Heat a skillet over medium-low heat with the avocado oil and butter. Cook each sandwich 2 to 3 minutes per side, pressing gently, until golden. Cover the pan and keep cooking until the cheese is completely melted.
Carly’s Expert Tips
- Don’t rush the onions. Low heat and time are what make them sweet and jammy instead of burnt.
- Add a splash of water if the onions start sticking to the pan.
- Cover to melt. A lid traps heat so the cheese melts before the bread gets too dark.
- Make a big batch of onions. They keep for days and are amazing on burgers and pizza.
Make It Your Own
- French onion style: Use all Gruyère and add a little fresh thyme to the onions.
- Add protein: Layer in thin slices of roast beef or crispy bacon.
- Apple: Add thin slices of apple for a sweet crunch.
- Spicy: Swap the cheddar for pepper jack and add a little hot honey.
Make-Ahead and Meal Prep: Caramelize the onions up to 5 days ahead and store them in the fridge, or freeze them in small portions for up to 3 months. With the onions ready, the sandwiches take about 10 minutes.
What to Serve With Caramelized Onion Grilled Cheese
This grilled cheese was made for dunking into Roasted Tomato Basil Soup. It is also great with a crisp green salad, kettle chips or a bowl of Chicken Pot Pie Soup.
Storing and Reheating
Grilled cheese is best fresh off the skillet. Store extra caramelized onions in an airtight container in the fridge for up to 5 days.
To reheat a leftover sandwich, warm it in a skillet over low heat or in the air fryer at 350°F for 3 to 4 minutes.
What People Ask About Caramelized Onion Grilled Cheese
How long does it take to caramelize onions for grilled cheese?
Plan on 40 to 50 minutes over medium-low heat. Rushing them on high heat burns them before they get sweet.
What cheese is best for caramelized onion grilled cheese?
Sharp cheddar and Gruyère are my favorite combo. Swiss, fontina and provolone also melt beautifully.
Why put mayo on the outside of grilled cheese?
Mayo browns evenly and creates a crisp, golden crust without burning as quickly as butter.
Can I make caramelized onions ahead?
Yes. They keep in the fridge for up to 5 days and freeze well too.
Caramelized Onion Grilled Cheese
Ingredients
Method
- Heat butter and olive oil in a skillet over medium-low heat. Add the sliced onions and season with salt and pepper.
- Cook for 40-50 minutes, stirring frequently, until onions are deeply golden and jammy. During the last 10 minutes, stir in balsamic vinegar and continue cooking.
- Spread mayo on one side of each slice of bread. Flip the bread over and spread Dijon on the other side.
- Divide the cheddar, Gruyère, and caramelized onions between the two sandwiches. Top with the remaining bread, mayo side facing out.
- Heat a skillet over medium-low heat and add avocado oil or butter. Cook each sandwich for 2-3 minutes per side, gently pressing with a spatula. Once the bread is golden, cover the pan and continue cooking until the cheese is completely melted.
