Preheat your oven to 400°F (200°C). Line a baking sheet with parchment paper for easy cleanup.
Clean your mushrooms with a damp paper towel - avoid washing them under running water as they'll absorb too much moisture.
In a large bowl, combine olive oil, minced garlic, thyme, salt, and pepper.
Add the mushrooms to the bowl and toss until they're evenly coated with the oil mixture.
Arrange the mushrooms on the prepared baking sheet, spacing them evenly.
Roast for 15 minutes, then remove from the oven and sprinkle with Parmesan cheese.
Return to the oven for an additional 5-7 minutes, or until the cheese is melted and slightly golden.
If using butter, drizzle it over the hot mushrooms right after they come out of the oven.
Garnish with fresh parsley and serve immediately.