Preheat oven to 350°F and generously spray a 12-cup muffin pan with cooking spray.
In a food processor pulse oats until fine and flourlike. Add walnuts and pulse until blended with oat flour. Pulse in olive oil, salt, and water until mixture comes together.
Spoon walnut-oat crust mixture evenly (about 1¼ tablespoons each) into each muffin cup and press into bottom and partially up sides. Bake 25 minutes until crust begins to brown. Cool on a wire rack 10 minutes.
In a large bowl whisk eggs, garlic powder, oregano, and mustard until well combined. Stir in spinach, sweet pepper, and cheese.
Fill each muffin cup evenly with egg-veggie mixture and bake 20 minutes or until eggs are set. Cool slightly on wire rack before removing egg cups by carefully inserting a sharp knife between crust and muffin pan. Makes 12 servings.