Preheat the oven to 350 degrees F (175 degrees C). Grease an 8-inch square metal baking dish.
Place bacon in a large skillet and cook over medium-high heat, turning occasionally, until evenly browned, about 10 minutes. Drain bacon slices on paper towels and let cool slightly, about 5 minutes; break into bite-size pieces.
Heat butter in a large skillet over medium heat. Cook cauliflower, bell pepper, and green onions in the hot butter until tender, 5 to 7 minutes. Remove from heat and set aside.
Beat egg whites, water, oregano, black pepper, and garlic powder together in a mixing bowl until foamy. Stir in cooked bacon, mozzarella cheese, mushrooms, Parmesan cheese, tomato, and cooked cauliflower mixture. Pour into the prepared baking dish.
Bake in the preheated oven until set, about 40 minutes.