Preheat the broiler. Slice a baguette into 24 slices. Broil on a baking sheet until golden, 3 minutes. In a skillet, heat 2 tablespoons extra-virgin olive oil over medium heat. Cook 1 thinly sliced onion and 1 chopped clove garlic for 5 minutes. Add 1 pound sliced cremini mushrooms and 1 tablespoon fresh thyme and cook until softened, 10 minutes; season with salt and pepper. Top the toasts with the mushrooms and 1/4 pound shredded gruyere cheese and broil for 2 minutes.