Heat the olive oil (2 tablespoons) in a medium oven-safe skillet (such as a stainless steel or a well-seasoned cast iron skillet) over medium high heat.
Sauté the sliced red onion (¼ cup) in the oil until softened and starting to brown, about 3 minutes.
Add the arugula (2 cups packed), season with salt and pepper to taste, and turn the heat down to medium. Stir until arugula has wilted, about 2 minutes.
Stir in cooked salmon (4 oz.), feta (4 oz.), and 8 beaten eggs and season again with salt and pepper, if needed.
Keep stirring for about one minute, until eggs begin to set a bit.
Place under the broiler for 6-8 minutes, or until eggs have set and top of frittata is golden brown.
