Toast the baguette slices in the oven for about 10 minutes at 375° F(190°C), or in the air fryer for 3 minutes. Drizzle 1 tablespoon of olive oil over the crostini.
Heat a pan over medium heat and add the butter. Once melted, add the onions, mushrooms, salt and pepper to the pan and saute until golden, about 8-10 minutes. Add the garlic and saute for 1 minute. Remove from heat.
Top each crostini with mushroom mixture, grated Parmesan, and a sprinkle of thyme.