Preheat the oven: Set your oven to 200°C (400°F). Line the tray with parchment paper before you put any ingredients in it.
Prepare the mushrooms: Place the cleaned mushrooms in a large mixing bowl.
Make the garlic butter sauce: In a small bowl, mix the melted butter, olive oil, minced garlic, thyme, salt, and black pepper.
Toss the mushrooms: Pour the garlic butter sauce over the mushrooms. Toss until all the mushrooms are evenly coated.
Arrange on the tray: Spread the mushrooms in a single layer on the prepared baking dish.
Bake: Bake in the preheated oven for 15–20 minutes, stirring halfway through, until the mushrooms are tender and golden.
Optional topping: In the last 5 minutes of baking, sprinkle parmesan cheese over the mushrooms for a cheesy finish.
Garnish and serve: Remove from the oven, sprinkle with fresh parsley, and serve warm.