Add the steamed asparagus to the skillet and season with sea salt and black pepper. Toss together and turn off the heat.
In a bowl, whisk together 6 whole eggs, 1/2 cup pre-separated egg whites, and 1/4 cup milk.
Pour the egg mixture into the skillet with the asparagus and onions.
Sprinkle 2 ounces of shredded Swiss cheese and 2 ounces of crumbled feta cheese over the top of the frittata.
Bake in the preheated oven for 25 minutes or until the frittata is set and golden brown around the edges.
Let cool to room temperature before slicing and serving. Enjoy!
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