Ingredients can vary....like most of my recipes. My ingredients were artisan bread, mushrooms, yellow onions, swiss cheese, tomatoes, low fat mayo. You can add ANYTHING your heart desires : olives, deli meats, bacon...the list goes on and on.
Saute the onions on low...sweat them and you don't want to brown them...just until they're almost translucent. you can use olive oil, butter, margarine....again your choice.
When the onions are almost translucent, add your sliced mushrooms...salt and pepper or season to your taste. When the mushrooms are cooked to your liking (I like them still firm...some like them totally cooked) drain the pan and store in a bowl, covered.
Now time to build the melt...mayo, cheese, tomatoes, mushrooms and onions. Then it's toasting time! Cook in a skillet...again with olive oil...butter...whatever your choice is. And when flipping (since it's such a stacked sandwich, I lifted the skillet to meet halfway with the sandwich during flipping. All four of my sandwiches stayed together. I even got a pat on my back from my daughter, "Nice job mom!"
And plate it! We paired our melts with baked chedder cheese chips.