Use a spoon to remove the gills from the underside of the mushrooms. Put the mushrooms in a small baking dish.
Combine the oil, red pepper flakes, sugar, thyme and garlic in a small saucepot and heat until bubbling and aromatic. Pour the warm oil over the mushrooms, add 1/4 teaspoon salt and toss gently to coat. Cover and marinate at room temperature for at least 1 hour and up to overnight.
Preheat the oven to 425 degrees F. Arrange the mushrooms on a baking pan and roast until browned and tender, about 30 minutes. Transfer the mushrooms to a serving plate and drizzle with the liquid from the pan.