Set the oven to 400°F (200°C). Line a sheet pan with parchment or lightly grease with oil or spray.
Clean mushrooms with a damp cloth. Trim stems and halve larger ones.
Mix melted butter, garlic, thyme, salt, pepper, and red pepper flakes (if using) in a small bowl.
Add mushrooms to the pan, pour the sauce over, and toss to coat. Spread in a single layer.
Bake for 20–25 minutes, flipping halfway. They should be golden, tender, and juicy.
Drizzle with lemon juice (optional) and top with parsley. Serve warm as a side or over grains/toast.