Heat the oven: Preheat to 375°F (190°C). Lightly grease a standard 12-cup muffin tin with cooking spray or brush with oil. Make sure the sides are coated well to prevent sticking.
Prep the mix-ins: Dice the ham and chop the veggies into very small pieces.
Smaller pieces distribute better and help the muffins hold together.
Whisk the eggs: In a large bowl, whisk eggs, milk, Dijon, garlic powder, onion powder, salt, and pepper until smooth and slightly frothy. The milk and whisking help keep the texture tender.
Add the fillings: Stir in the ham, cheese, bell pepper, green onions, and parsley. The mixture will look full—this is what gives each muffin plenty of flavor.
Portion the batter: Divide evenly among the 12 muffin cups.
Fill each cup about 3/4 full. Give the tin a gentle tap on the counter to level the mixture.
Bake: Place on the middle rack and bake for 16–20 minutes, or until the centers are just set and a toothpick comes out mostly clean. They will puff in the oven and settle slightly as they cool.
Cool briefly: Let the muffins rest in the pan for 3–5 minutes.
Run a thin knife around the edges if needed to loosen them.
Serve or store: Enjoy warm, or cool completely on a rack before refrigerating or freezing. Add a sprinkle of extra cheese on top while hot if you want a melty finish.