1Heat oven to 450°F. Spray 8-inch square (2-quart) glass baking dish with cooking spray.
2In baking dish, gently mix mushrooms, butter, garlic, thyme and pepper.
3Roast uncovered 30 minutes, stirring gently halfway through baking.
4Spread 1 quarter of avocado over each slice of toasted bread. Top with roasted mushroom mixture. Serve immediately.