Preheat your oven to 350°F (175°C) and lightly grease 6 slots of a standard muffin tin with avocado oil.
Place a skillet over medium heat and add the ground turkey sausage, breaking it apart with a spatula until fully browned and cooked through.
Roughly chop the fresh baby spinach and set it aside.
In a large mixing bowl, whisk together the whole eggs, liquid egg whites, nutritional yeast, sea salt, black pepper, and garlic powder until the mixture is well combined and slightly frothy.
Stir the cooked turkey sausage, chopped spinach, and crumbled feta cheese into the egg mixture.
Carefully pour the mixture into the prepared muffin tin, filling each cup nearly to the top.
Bake for 20 to 25 minutes, or until the egg cups are set in the center and slightly golden on the edges.
Allow the cups to cool for 5 minutes before using a knife to gently loosen the edges and pop them out of the tin.