Preheat the oven to 400 degrees . In a large skillet, combine the oil, onion and garlic and cook on medium-high heat, stirring occasionally, for 2 minutes. Add the mushrooms and cook until tender, about 5 minutes. Stir in the tomato sauce and olives and bring to a simmer.
Spread the sauce mixture in a greased 2-quart baking dish. Arrange the pepperoni in a single layer on top and sprinkle with the cheese. Top with the biscuit pieces. Bake until the cheese is bubbly and the biscuits are browned, about 15 minutes. Let cool for about 5 minutes before serving.