1. Preheat your oven to 350 F (175 C). Lightly grease a 12-cup muffin tin with cooking spray or a small amount of oil. Ensuring the tin is well-greased will help prevent the egg cups from sticking and make for easy removal.

2. In a large mixing bowl, crack the 12 large eggs. Add the 1/2 cup of milk, along with salt and black pepper to taste. Whisk vigorously until the egg mixture is well combined and slightly frothy. This ensures a light and airy texture for your egg cups.

3. Stir in the shredded cheddar cheese, finely chopped bell pepper, finely chopped onion, and the cooked and crumbled sausage or bacon (if using) into the egg mixture. Gently fold everything together until the ingredients are evenly distributed throughout the eggs.

4. Carefully pour the egg mixture into each prepared muffin cup, filling them about two-thirds full. Be mindful not to overfill, as the eggs will puff up slightly during baking.

5. Place the muffin tin in the preheated oven and bake for 15-20 minutes. The egg cups are done when they are set in the center, lightly golden around the edges, and spring back when gently touched. A toothpick inserted into the center should come out clean.

6. Once baked, remove the muffin tin from the oven. Let the egg cups cool in the tin for a few minutes before carefully running a knife around the edges and removing them. Serve warm, or allow them to cool completely before storing.