1. Preheat your oven to 350 F (175 C). Lightly grease a 12-cup muffin tin or line it with paper liners.

2. Heat the olive oil in a large skillet over medium heat. Add the chopped bell peppers and onion and cook for 5-7 minutes, or until they are softened and slightly translucent. If using spinach or mushrooms, add them in the last 2-3 minutes of cooking until wilted.

3. In a large bowl, whisk together the eggs, milk, salt, and black pepper until well combined and slightly frothy.

4. Evenly distribute the cooked sausage, sauteed vegetables, and half of the shredded cheddar cheese among the prepared muffin cups.

5. Carefully pour the egg mixture over the ingredients in each muffin cup, filling them about two-thirds full. Sprinkle the remaining shredded cheddar cheese over the top of each cup.

6. Bake for 20-25 minutes, or until the egg cups are set in the center and lightly golden brown on top. A toothpick inserted into the center should come out clean.

7. Remove the muffin tin from the oven and let the egg cups cool in the tin for a few minutes before transferring them to a wire rack to cool completely.