Sauté Vegetables: First, preheat the oven to 340 degrees Fahrenheit. Heat the oil in a large skillet over medium heat, then add the diced onion. Cook for two to three minutes or until translucent.
Transfer to Baking Dish: Grease a 7×11-inch casserole dish with nonstick spray so the eggs do not stick to the sides. Drain the excess liquids from the cooked vegetables.
Bake: Top the eggs with feta and cheddar cheese. Then, bake this easy keto breakfast casserole in the preheated oven for 30-45 minutes until cooked through completely.