Preheat the oven to 375°F. Lay baguette slices on a large baking sheet and drizzle with olive oil. Flip the slices over, drizze again lightly, and push them around on the baking sheet to spread the olive oil. Salt and pepper one side. Bake for 5-7 minutes, flipping halfway through. Once crostini is toasted and golden, remove from oven, and preheat the oven to 525°F on broil. Top each crostini evenly with the shredded cheese. Broil on the middle rack for 3-5 minutes until cheese is melted and golden (watch carefully so it doesn't burn!).
Meanwhile, in a large skillet, add the chopped mushrooms and cook over medium heat alone for 8-10 minutes until the liquid has evaporated (they will release a lot of water during this process). Stir the mushrooms occasionally; it's ok if some stick, it'll be scraped up later.
Once the excess water is evaporated, add in the butter and let it melt. Then, add the shallot and cook for about two minutes, beginning to add your salt in layers. Add the garlic, red pepper flakes, black pepper, and again salt in layers, then cook for an additional 4-5 minutes, stirring as needed. Stir in the thyme, cooking for another 1-2 minutes until fragrant. Remember to taste test for desired salt levels! Remove from heat and set aside.
Top each crostini with the mushroom mixture. Garnish with sprouts or extra thyme as desired.