- With the rack in the middle position, preheat the oven to 220 °C (425 °F).
- Place the bread slices on a baking sheet. Set aside.
- In a skillet, gently cook the onion in 15 ml (1 tablespoon) of oil until caramelized. Set aside. Add the mushrooms and sauté over high heat until golden brown. Season with salt and pepper. Add the reserved onion and stir to combine.
- Spread the mushroom mixture on the bread slices. Sprinkle with the cheese and bake for 8 to 10 minutes. If necessary, finish cooking under the broiler until the cheese is golden brown.