6ozprovolone cheesesliced (4 slices, 2 slices per sandwich)
Add two tablespoons of unsalted butter to a large heated skillet. Add sliced red onions and mushrooms. Cook over medium-high heat for 5 minutes, stirring occasionally.
Add the red wine vinegar and stir to combine. Let cook an additional 3-5 minutes until mushrooms and onions are soft. Remove from heat and place mushrooms and onions in a bowl. Add salt and pepper to taste.
Place 8 slices of Schar white bread on a flat surface. Spread the remaining two tablespoons of butter on one side of each slice. Flip the bread over.
Place a piece of provolone cheese on each piece of bread.
On 4 of the slices of bread, spoon the mushroom and onion mixture over the cheese. Top with some fresh spinach.
Place remaining 4 slices of bread on top of the spinach, cheese side down (butter side up).
In a large, preheated skillet over medium heat, place the sandwiches butter side down. Cook for 3-4 minutes on each side, until the bread is golden brown and the cheese is melted. Cut in half and serve.
Find a good gluten-free bread to use for this sandwich. You want one that won't fall apart and crumble and one that tastes amazing too. I used Schär artisan baker white bread for this sandwich and it worked really well!
Use softened butter so it easily spreads on the bread.
You can substitute any easy melting cheese for the provolone. This sandwich will work with cheddar, Swiss and gouda cheeses as well.
I used baby portobello mushrooms, but cremini or trumpet mushrooms would also be delicious.
If you don’t have a red onion, yellow or vidalia onions are also perfect. Use whatever you have. I like red onions because they have a little more bite.
The red wine vinegar gives the mushrooms and onions a little flavor and helps soften them. You can use sherry vinegar if you want to play around with flavors.
Make sure your skillet isn’t too hot so that the bread does not burn before the cheese melts. You want it to melt at a slow and steady pace.
The secret to adding ingredients like mushrooms and onions to your grilled cheese is to make sure there is a slice of cheese on the bottom and top of your filling so that it all gets encased in cheesy goodness and doesn't make your bread soggy.