In a big skillet, warm the avocado oil over medium-high heat. Add the minced garlic, chopped red bell pepper, and chopped onion. Sauté for about 5 minutes, stirring periodically, or until the onion has softened.
Add the chopped cooked sausage links and baby spinach to the skillet. Cook, stirring periodically, for about 8 minutes, or until the mixture has evaporated any excess liquid.
Set a muffin tin with paper or silicone liners and preheat the oven to 350 degrees. If you would rather cut waste or don't have muffin liners, you can also use nonstick cooking spray on the muffin tin.
Once the eggs are cracked into a sizable bowl, beat them thoroughly with a whisk. Transfer the cheese, if using, to the bowl containing the beaten eggs and well mix to incorporate.
Note: Because it makes pouring easier, I prefer to whisk the eggs in a 4-cup measuring cup with a spout. I suggest using a measuring cup or mixing bowl with a spout if you have one so that pouring the egg mixture will be simple.
Fill the holes in the muffin tray ½ to ⅔ of the way up with the vegetable and sausage mixture. Fill the remaining space in each muffin hole with eggs by pouring the egg mixture into them.
Bake for 18 to 22 minutes, or until the eggs are thoroughly done, on the center rack of a preheated oven. Put a meat thermometer in the middle of an egg muffin to check for doneness.
When an egg reaches an internal temperature of 160 degrees Fahrenheit, it is considered fully cooked. Let the egg cups cool completely before removing the muffin papers to consume one.