I saw a similar recipe on Pinterest (via a blog). I simplified the recipe considerably. Here it is-enjoy!
1/2 T butter (plus more for buttering the bread)
1 T olive oil
1 small onion,
8 oz Crimini mushrooms (plain-ol' white ones)
2 cloves garlic
1 tsp thyme chopped
salt and pepper to season
1 T parsley chopped
1 cup Fontina (or Gruyere) cheese, torn in pieces
1/4 cup Parmesan/Reggiano cheese, shredded
4 pieces bread
Saute mushrooms and onion in olive oil and 1/2 T butter. After they start to get tender, add garlic. Take pan off heat and add herbs. Salt and pepper to taste. Mix in both cheeses and set aside. Butter outsides of bread and add to the pan. Divide mushroom/cheese mixture among the 2 sandwiches and grill on both sides until golden brown and cheese is melted.