Preheat and prep: Preheat your oven to 425°F (218°C) if your skillet is oven-safe; otherwise plan to cover the skillet on the stove. Rinse the inside of a 9-inch nonstick or well-seasoned oven-safe skillet and have a spatula ready. Preheating the oven ensures a quick finish to set the top without overcooking the interior.

Gently sweat the onion and garlic: Warm the skillet over medium-low heat and add the 1 tsp butter (or 1/2 tsp butter + 1/2 tsp olive oil for a higher smoke point). Add the diced onion and a pinch of salt. Cook 3–4 minutes until the onion is translucent and fragrant, stirring occasionally. Add the minced garlic for the last 30 seconds. Sweating the aromatics on low heat softens them and draws out sweetness without browning, which keeps the flavor delicate and suited to the light egg base.

Combine the eggs and cottage cheese: In a medium bowl whisk together the 8 egg whites and 1 whole egg until slightly frothy. Stir in the 1/2 cup low-fat cottage cheese, 1/4 cup shredded part‑skim mozzarella, 1/2 tsp kosher salt and 1/4 tsp black pepper. Whisking briefly introduces a little air for lift and the cottage cheese melts into the whites to give creamy texture and a protein boost without heavy fat.

Taste and adjust seasoning: Take a tiny spoonful of the raw mixture and taste (or taste a drop in the pan if you prefer cooked). Adjust salt and pepper — eggs with cottage cheese need a bit more seasoning than plain whites because dairy can mute saltiness. Good seasoning at this stage means less need to add salt later.

Pour and gently cook: Increase heat to medium-low. Pour the egg-cottage-cheese mix into the skillet over the softened onions. Use a spatula to distribute onions evenly. Let the mixture cook undisturbed for 2–3 minutes until the edges begin to set. Cooking gently at low heat prevents the egg whites from tightening and becoming rubbery — slow coagulation yields tender curds.

Finish to set the top: Option A (oven-safe skillet): Transfer the skillet to the preheated 425°F oven for 4–6 minutes until the top is set and lightly golden. Option B (stovetop only): Reduce heat to low, cover the skillet and steam for 6–8 minutes until set. The brief high heat from the oven or the gentle steam under a lid finishes the top quickly without drying out the interior.

Rest and garnish: Remove the skillet from heat and let rest 2 minutes — resting lets carryover heat finish cooking and the protein matrix relax so slices hold together. Scatter the chopped parsley over the top and let it wilt slightly from residual heat.

Serve: Slide the frittata onto plates or cut into wedges in the skillet. This recipe serves two generously; pair with a crisp green side if desired. The result is a tender, moist, high-protein egg bake with melty pockets of cheese and gentle sweet notes from the sweated onion.