Preheat oven to 350 degrees F.
On a baking sheet or oven proof pan, drizzle sliced onion and mushrooms with extra virgin olive oil.
Bake in the oven for about 20 minutes until onions are a golden brown or clear.
Heat a medium-sized cast iron skillet over medium heat on the stove top for five minutes before vegetables are done roasting. I used a #8 Griswold skillet.
While the pan is heating, put butter slices on four sides of bread.
When vegetables are completed put two pieces of buttered bread face down in skillet and add the vegetables and Gouda cheese and top with bread.
Cook for about 3-4 minutes and gently flip with a metal spatula and continue to cook the other side an additional 3-4 minutes or until the bread is a golden brown and the cheese has melted.
Remove from stove and let cool about a minute. Slice in half with a sharp knife.
I served the sandwiches with a bowl of hot tomato bisque soup. This made a wonderful meal for my wife and me.