Heat the avocado oil in a large skillet over medium-high heat. Add the chopped onion, chopped red bell pepper, and minced garlic and sauté, stirring occasionally, until the onion has softened, about 5 minutes.

Transfer the chopped cooked sausage links and the baby spinach to the skillet and cook, stirring occasionally, until any excess liquid has evaporated from the mixture, about 8 minutes.

Preheat the oven to 350 degrees Fahrenheit and line a muffin tin with paper liners or silicone liners. You can also spray the muffin tray with nonstick cooking spray if you don’t have muffin liners or if you prefer to reduce waste.

Crack the eggs into a large bowl and whisk until they are well-beaten. If you’re adding cheese, transfer it to the bowl with the beaten eggs and mix well until combined. Note: I like mixing the eggs in a 4-cup measuring cup with a spout, because it facilitates pouring. If you have a mixing bowl with a spout or a large enough measuring cup, I recommend using it so that the egg mixture is easy to pour.

Transfer the vegetable and sausage mixture to the prepared muffin tray and fill the holes ½ to ⅔ of the way up.

Pour the egg mixture into each muffin hole, filling the holes the rest of the way up with eggs.

Bake on the center rack of the preheated oven for 18 to 22 minutes, or until the eggs are cooked through. To verify for doneness, insert a meat thermometer into the center of an egg muffin. Eggs are considered fully cooked once they reach an internal temperature of 160 degrees Fahrenheit.

Allow the egg cups to cool completely before peeling off the muffin papers to enjoy one.