Preheat oven to 425 F. Toss mushrooms in 1 tablespoon of olive oil and season with salt and pepper. Spread them out on a greased or sprayed baking sheet. Roast for approximately 10-12 minutes, until beginning to brown. Remove from oven and cool to room temperature.
Meanwhile, place half the ricotta in the bottom of a greased casserole dish. Layer half the tomatoes and mushrooms on top. Drizzle with half of the remaining olive oil and season with salt, pepper, and oregano. Spread the remaining ricotta over the top of the veggies, and layer the remaining tomatoes and mushrooms over the top. Again, drizzle with olive oil and season with oregano, salt, and pepper. Top all with grated Parmesan.
Bake at 425 F for about 20-25 minutes, or until ricotta is beginning to bubble.
Remove from oven and allow to sit for 10 minutes to set up. Serve with meat, baguette slices, or as a side dish all by itself along with slices of grilled rustic Italian bread.