These egg white muffins make a gorgeous brunch dish or a quick weekday breakfast. You can make them in advance and refrigerate them - they're excellent when quickly reheated in the microwave or even cold.
Preheat the oven to 400°F. Grease two 6-ounce ramekins with butter and place them on a rimmed foil-lined baking dish (to catch any spills).
In a medium bowl, whisk the egg whites until light and frothy. Add the salt, pepper, garlic powder, and parmesan, and whisk to incorporate.
Mix in the green onions (and any other additions, such as tomatoes) with a spatula. Pour the mixture into a spouted measuring cup and from the measuring cup into the greased ramekins. The ramekins should be ¾ full - if the mixture comes higher than that, use one extra ramekin to prevent spilling.
Place the ramekins on the foil-lined baking dish. Bake them until they are puffed and browned and a knife inserted in their center comes out clean, 20-25 minutes. Serve immediately.
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Notes
You can definitely vary your cheese. Parmesan is just so flavorful, so I like to use it, but sharp cheddar is great, too.
As for what to add to these muffins, I usually opt for scallions. But you can add mushrooms, cubed ham, sun-dried tomatoes... anything, really. Whatever you add to the muffins, try to limit the mix-ins to about ¼ cup and certainly no more than ⅓ cup.
You can keep these muffins in the fridge, in an airtight container, for up to four days. Reheat them gently in the microwave at 50% power. They are also excellent when served cold.