In a large nonstick skillet, heat oil over medium heat. Arrange mushrooms in a single layer and top with onions. Cook, without stirring, for 5 minutes or until mushrooms become red-brown on one side. Turn and cook about 4 minutes. Add garlic; cook 1 minute. Stir in tomatoes and olives; set aside.
Break eggs into a large bowl; whisk to break yolks. Add milk, flour, baking powder, salt and pepper; whisk to combine (batter may still be lumpy). Pour into skillet over vegetable mixture and stir until eggs and vegetables are combined. Cook, uncovered, over medium heat until edges begin to set, about 2-3 minutes. Transfer to oven; bake at 350° for 30-40 minutes or until mixture is set and golden brown. Slide onto serving platter and cut into wedges.
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