Preheat your oven to 425°F.
In a large bowl, combine the halved mushrooms, olive oil, minced garlic, thyme, salt, and black pepper. Toss until the mushrooms are well coated.
Spread the mushroom mixture in a single layer on a baking sheet.
Roast in the preheated oven for 20-25 minutes, stirring halfway through, until the mushrooms are golden brown and tender.
If using balsamic vinegar, drizzle it over the mushrooms during the last 5 minutes of roasting.
Remove from the oven and let cool slightly before serving.