1. Prepare Your Baking Dish: Begin by preheating your oven to 350 F (175 C). Lightly grease a 9×13 inch baking dish or a 12-cup muffin tin with cooking spray or a thin layer of olive oil. This prevents sticking and ensures easy removal of your breakfast items.

2. Whisk the Egg Mixture: In a large mixing bowl, crack the 12 large eggs. Add the 1/2 cup of milk, salt, and black pepper. Whisk vigorously with a fork or a whisk until the egg whites and yolks are fully combined and the mixture is slightly frothy. This ensures an even consistency throughout your breakfast.

3. Combine Ingredients: Gently fold in the cooked, diced chicken breast (or turkey sausage), shredded cheddar cheese, chopped bell peppers, and chopped spinach into the egg mixture. Stir until all the ingredients are evenly distributed. This step is where all the flavors come together.

4. Fill the Baking Dish or Muffin Tin: Carefully pour the egg mixture into your prepared baking dish, spreading it evenly. If using a muffin tin, fill each cup about two-thirds full. Don’t overfill, as the eggs will puff up slightly during baking.

5. Bake Until Set: Place the baking dish or muffin tin into the preheated oven. Bake for 15-20 minutes for muffin cups, or 20-25 minutes for a larger casserole, or until the eggs are fully set in the center and lightly golden brown around the edges. A visual cue is when a knife inserted into the center comes out clean.

6. Cool and Serve: Once baked, remove the dish from the oven and let it cool for 5-10 minutes before slicing or removing from the muffin tin. This allows the breakfast to firm up slightly, making it easier to serve.