Preheat oven to 400 degrees. Melt the butter in a large saute pan, cook the onions over medium heat until they are brown and sweet, about 15 minutes. Add the mushrooms, and cook until tender and all the juices are cooked out, about 10 minutes. Season well with salt and pepper. Sprinkle the surface of the tart round with the 2 tablespoons of bread crumbs, this will help absorb any excess liquid. Spoon the mushrooms on top of the tart circle, flatten out within 1-inch of the edge. In a small bowl combine the 1/4 cup bread crumbs, cheese, and basil. Sprinkle this on top of the mushroom mixture. Place in the oven and brown, about 5 to 8 minutes. Drizzle with olive oil.