01 - Preheat oven to 375°F (190°C). Grease a medium baking dish (about 8x8 inches or 2 liters) with butter or non-stick spray.
02 - In a large, clean bowl, beat the egg whites with an electric mixer on medium-high speed until stiff peaks form. Set aside.
03 - In a separate bowl, whisk together the egg yolks, cottage cheese, milk or cream, salt, black pepper, and sweet paprika until well combined and smooth.
04 - Stir in the shredded cheddar, grated Gruyère or mozzarella, sliced green onions, and optional spinach or broccoli and chives until evenly distributed.
05 - Gently fold the whipped egg whites into the cottage cheese mixture in two stages, using a spatula with slow, sweeping motions to preserve as much air as possible.
06 - Pour the fluffy mixture into the prepared baking dish and spread evenly, leveling the top with a spatula.
07 - Bake for 25 minutes, or until the top is golden brown, the center is set, and the casserole has puffed up noticeably.
08 - Remove from the oven and let rest for 5 minutes before slicing into portions and serving warm.