Preheat your oven to 400 degrees 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 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 puffed, browned, and a knife inserted in the center comes out clean, 20-25 minutes.
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Notes
You can definitely vary your cheese. Parmesan is just so flavorful, so I like to use it, but I'm sure sharp cheddar would be 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.