Whisk together eggs, milk and salt. In a large nonstick skillet, heat 2 teaspoons oil over medium-high heat; saute mushrooms and onion until golden brown, 5-6 minutes. Stir in spinach until wilted; remove from pan.
In same pan, heat remaining oil over medium-low heat. Pour in egg mixture. As eggs set, push cooked portions toward the center, letting uncooked eggs flow underneath. When eggs are thickened and no liquid egg remains, spoon vegetables on 1 side; sprinkle with cheese and olives. Fold to close; cut in half to serve. Sprinkle with pepper.