Preheat oven to 350°F (175°C) or use a toaster. Toast sourdough slices 3-4 minutes until golden and crisp. Set aside on a plate.
3 slices sourdough bread
Pat mushrooms dry with paper towels to remove moisture, then slice them ¼-inch thick. Mince garlic cloves and set aside separately.
8 oz button mushrooms, 2 cloves garlic
Heat 2 tablespoons olive oil in a cast iron skillet over medium-high heat. Add mushrooms in a single layer and cook 3 minutes without stirring to brown the bottoms. Add salt and black pepper, then stir and cook another 3-4 minutes until mushrooms are golden and tender.
8 oz button mushrooms, 2 tbsp olive oil, to taste salt, to taste black pepper
Push mushrooms to one side of the skillet. Add 1 tablespoon butter (if using) and minced garlic to the cleared space. Cook 1-2 minutes until garlic is fragrant but not browned. Stir everything together.
1 tbsp butter, 2 cloves garlic
Remove skillet from heat. Stir in chopped fresh thyme or parsley and an optional squeeze of lemon juice. Spoon mushrooms over toasted sourdough bread. Top with grated parmesan or feta if desired and serve immediately.
1 tbsp fresh thyme or parsley, 1 tsp lemon juice, to taste parmesan or feta, 3 slices sourdough bread, 8 oz button mushrooms