These buttery crispy sandwiches have the flavor of french onion soup with caramelized onions sandwiched with sauteed mushrooms, swiss cheese, and fresh thyme. This grilled cheese sandwich is the perfect lunch time indulgence!

French Onion Grilled Cheese Featured Image.

A Caramelized Onion Grilled Cheese Sandwich is always tasty, but add in some fried mushrooms and lunch has been brought to another level. Golden mushrooms and sweet caramelized onions are sandwiched in between two slices of bread with melty swiss cheese. This is a tasty sandwich recipe that you will want to enjoy often!

This grilled cheese sandwich is perfect for lunch or dinner and excellent with a bowl of soup on a chilly day, or a side of chips and garlic dill pickles.

A Caramelized Onion Grilled Cheese Sandwich grilled until golden brown with grill marks on the toasty bread, mushrooms caramelized onions and cheese showing with the sandwich cut in half and stacked on top of each other cut side facing out.

Celebrating Grilled Cheese Month

April is National Grilled Cheese Month. For me, that is a big thing, because we love grilled cheese! I will be sharing a few new sandwiches throughout the month that we have been enjoying and are in love with, including this Mushroom Caramelized Onion Grilled Cheese Sandwich.

Grilled Cheese Sandwiches are our go to when we are ready for lunch, but weren't prepared. I always keep caramelized onions in the fridge and the mushrooms are easy to fry up quickly.

Freshly grated swiss cheese on a brown cutting board on a marble countertop.

Ingredients Needed For Caramelized Onion Grilled Cheese

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A nice firm bread. I used a Rosemary Tuscan Pane bread that I bought from the grocery store bakery. I like how the rosemary bread pairs so well with the caramelized onion grilled cheese.

Caramelized Onions, these are super easy to make and this post tells you how to make caramelized onions in a quick and easy way!

Fried Mushrooms, I like to make this Sautéed Mushrooms with Umami.

Swiss Cheese, I like to buy the brick cheese and shred it for grilled cheese sandwiches. I find the cheese melts easier and faster when it is shredded.

Softened butter for coating the grilling side of the bread. I prefer to use salted butter but unsalted is fine as well.

Grill Pan, I love to use this grill pan to get the grill marks on my sandwich. I also like to use a wide spatula to flip the sandwich.

Box shredder to shred the swiss cheese.

Tips and Substitutions For The Best Grilled Cheese Sandwich

Make sure the bread you use is a good sturdy bread. I like to use the Tuscan Pane bread. It holds up well to any sandwich I have grilled. Any artisan bread that is nice and firm will work.
When turning the caramelized onion grilled cheese sandwich over to toast on the opposite side, be sure to use a wide thin spatula and use your other hand to help support the sandwich so it turns over without spilling the fillings out.
Use a small cast iron skillet to press the sandwich down some while it is cooking. The sandwich will look super tall, and you want the cheese to melt and bring the sandwich together.

A small cast iron skillet placed on top of the sandwich while it is grilling on the grill pan.

Substitute the swiss cheese for one of your favorite cheeses. Good cheeses to use would be brie, pepper jack, mozzarella, monterey jack, and a smoked jack cheese is excellent also!
Amp the sandwich up a little and add some protein to it. Ham or Bacon are good choices. My son likes to add pepperoni slices to his.
Substitute fried baby portobello mushrooms in place of the crimini.
Toast the inside of the sandwich before assembling it to grill, this will make it extra crispy and buttery.
Sprinkle some red pepper flakes into the caramelized onions when cooking to add a little bit of heat.

Caramelized Onion Grilled Cheese Sandwich with fried mushrooms grilled until golden with grill marks on the bread cut in half and stacked with one mushroom and caramelized onion on the plate that fell out of the sandwich.

Other Sandwich Recipes To Try

Bacon Tomato Grilled Cheese

Cheesesteak Sandwich

Crispy Chicken Sandwiches

Grilled Cheese Sandwich with Bacon, Jalapeno, and Rhubarb Sauce

Grilled Ham and Blue Cheese Sandwich

Loose Meat Sandwiches

Pimento Cheese Tea Sandwiches

Shrimp and Avocado Chaffle Sandwich

Slow Cooker BBQ Beef Brisket Sandwich

Slow Cooker Pulled Pork Sandwiches

Steak Sandwich with Blue Cheese Spread

A Caramelized Onion Grilled Cheese Sandwich cut in half and stacked with the mushrooms on the inside of the sandwich visible and one mushroom and caramelized onion on the plate.

*If you made this Caramelized Onion Grilled Cheese Sandwich with Mushrooms, please give it a star rating*

📖 Recipe