1. Adjust oven rack to middle position and heat oven to 350°F. Spray 13 x 9-inch baking dish with vegetable oil spray. Heat oil in 12-inch nonstick skillet over medium-high heat until shimmering. Add potatoes, mushrooms, onions, bell pepper, and 1/2 teaspoons salt; cover; and cook for 5 minutes. Uncover and continue to cook until liquid has evaporated and vegetables are tender, about 10 minutes.

2. Transfer vegetable mixture to prepared dish and let cool for 10 minutes. Stir in bacon, scallions, and Cheddar. Whisk eggs, milk, cornstarch, 1/2 teaspoon pepper, and 1/4 teaspoon salt together in large bowl. Pour egg mixture over vegetable mixture. Sprinkle casserole with Parmesan. Bake until eggs have set and top is lightly browned, 30 to 40 minutes, rotating dish halfway through baking. Let cool for 20 minutes before serving.

TO MAKE AHEAD: Cooked, cooled vegetable and egg mixtures can be refrigerated separately for up to 24 hours.

Serves 8