Heat oven to 350ºF and generously spray 12-cup muffin pan with nonstick cooking spray.
In bowl of food processor, pulse oats until fine and flour-like. 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 1 1/4 tablespoons each) into each muffin cup and press into bottom and partially up sides. Bake 25 minutes until crust begins to brown. Cool on wire rack 10 minutes.
In large bowl, whisk eggs, garlic powder, oregano and mustard until well combined. Stir in spinach, bell 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 sharp knife between crust and muffin pan.