Since we already broke into a package of feta, this frittata takes care of the rest of it! Feta adds a savory layer to the frittata. Paired with an avocado citrus salad, this would make a great brunch for company too. Use the leftovers as breakfast sandwich fillings!
Heat up a skillet over medium heat. Add in cooking oil and then onions to heated oil with a dash of salt. Saute for about ~5 minutes, until softened. Next add in mushrooms and cook for another 3 to 5 minutes, to let mushrooms soften.
Add in spinach leaves and toss through to let the heat wilt the spinach. Pour in egg and feta mixture. Stir to mix everything through and then transfer to oven. Cook for 18 to 22 minutes until knife comes out cleanly.
While frittata is cooking, dice shallots (your vinaigrette may have already been made yesterday). Whisk with Dijon, vinegar, and olive oil. Season to taste with salt and pepper.
Supreme (that's just the fancy French word for segmenting orange slices, but if you use it, people will think you're so cool) citrus and cube avocado. Toss vinaigrette with citrus, avocado, and greens, and enjoy with a warm slice of frittata!