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 (1/4 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 4 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.
Make it paleo/whole30/dairy free: omit the feta cheese.
The provided nutrition information does not include any added sodium from seasoning to taste, any optional ingredients, and it does not take brands into account. Feel free to calculate it yourself using this calculator or by adding the recipe to Yummly.