Preheat oven to 425 degrees F.
Season mushrooms: Place mushrooms and onions on a large, rimmed baking dish. Toss with oil and season with a pinch of salt and pepper. Arrange the vegetables in a single layer, with the mushrooms facing stem side down. Add 4 sprigs of fresh thyme and 2 sprigs of rosemary to the baking dish.
Partially roast: Place mushrooms and onions in the oven. Roast for 15 minutes or until lightly browned.
While the mushrooms are roasting, make the butter sauce: (SEE NOTES) In a large, microwave-safe liquid measuring cup, combine butter, wine, garlic, thyme and oregano. Season with ¼ teaspoon salt and 1/8 teaspoon pepper. Microwave at 1 for 1 minute. Stir and microwave again for 1 ½ in 30 second increments.
Toss with butter and finish roasting: Remove mushrooms and onions from the oven. Drizzle the butter sauce over the vegetables and gently toss or stir to coat. Continue roasting for 5-10 minutes or until mushrooms are browned and tender. Taste and adjust for seasoning with salt and pepper.
Serve: Transfer mushrooms and onions to a serving dish. Drizzle with butter sauce from the baking dish. Garnish with fresh herbs and lemon if using. Enjoy!
Nutritional information is an approximation and based upon 6 servings.