Preheat oven to 375F. Line a baking sheet with parchment paper. Add baguette slices to the baking sheet.
Drizzle olive oil over the baguette slices, just making sure to get a drizzle on each slice. Sprinkle some salt over top. Bake for 8-10 minutes, or until lightly golden on the edges.
In a large skillet over medium-high heat, melt the butter. Add the mushrooms and spread out into an even layer. Let cook, undisturbed, for about 5 minutes, or until browned. Give it a stir and cook for 1-2 more minutes, or until cooked through. Stir in the thyme, garlic, and salt and cook for 2 more minutes, stirring often. Remove from heat.
Top each toast with mushrooms (the amount depends on how big your toasts are, just add enough mushrooms to mostly cover it). Sprinkle cheese over top. Top with some extra chopped thyme if you have it and some black pepper. Place back into the oven until the cheese is melted, about 5 minutes.