Preheat the oven to 350 degrees F (175 degrees C). Spray a casserole dish with nonstick cooking spray.
Whisk egg whites, yogurt, salt, and pepper together in a bowl. Pour into the prepared dish.
Bake in the preheated oven until starting to set, about 10 minutes.
Meanwhile, saute mushrooms and onion in a pan until softened, 5 to 10 minutes. Spread mushroom mixture, sausage crumbles, and tomato over the parbaked egg layer. Cover with Cheddar cheese.
Continue baking until eggs are set and cheese is melted, 10 to 15 minutes.
Use any breakfast meat or switch up the type of cheese for a change from week to week.
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