Preheat oven to 350 degrees and grease a 12 cup muffin tin (I love using my silicone muffin liners for easy clean up)
Heat oil in a large non-stick skillet over medium high heat. Add sausage and cook, breaking up with the back of your spoon, until fully cooked through. Add onion, garlic, garlic powder, dried mustard, salt, and pepper, and sauté another few minutes until onion is soft; set this mixture aside.
In a medium sized bowl, whisk eggs and milk together and lightly season with salt and pepper.
Distribute sausage mixture evenly among the 12 muffin cups. Top with cheese and chopped spinach (pressing down to pack the cups if needed).
Next, evenly distribute egg mixture into the cups. Bake for 25-30 minutes, until eggs are set.
Store in an airtight container for 4 days. Enjoy!!