Wash the mushrooms and pat dry. Slice larger mushrooms in half. In a large bowl, add the mushrooms and quartered onion along with the olive oil, garlic powder, and kosher salt. Spread mushrooms and onion on baking sheet in a single layer. Add springs of fresh herbs.
Bake 10 to 12 minutes until the mushrooms are tender. Serve topped with chopped fresh rosemary and thyme or dried herbs.