Preheat oven to 400°F.
On a baking sheet toss sliced mushrooms and onion in olive oil. Sprinkle with a few pinches of salt and pepper. Bake in oven for about 20 minutes, or until roasted to preference.
In a skillet stove top, melt 4 tablespoons butter over medium heat.
While butter is melting, assemble sandwiches. On one slice of bread, layer shredded gouda, the roasted mushrooms and onions, then more gouda. (Layering this way ensures that the melty cheese will hold the sandwich together.) Salt and pepper to taste. Top with the other slice of bread. Lightly press together.
In the melted butter place one sandwich down for one second, then gently flip it over to the other side. Repeat with other sandwich.
Cook for about 2 minutes, until lightly browned, then gently flip sandwich on other side, and cook for an additional 2-3 minutes, or until cheese is melted and sandwich is browned to preference.
Recipe and Picture from shewearsmanyhats.com