Heat a medium skillet over medium heat. Add 1 teaspoon oil or a small pat of butter if needed. Add the diced onion and sauté until translucent, about 4–5 minutes.

Add peppers

Add the diced green and red bell peppers to the skillet and cook with the onion until softened, about 3–4 minutes more. Season lightly with a pinch of the salt and pepper. Remove from heat and set aside to cool slightly.

Prepare protein

If your ham is uncooked or bulky, warm it briefly in the skillet for 1–2 minutes to render edges and meld flavors. If using bacon or sausage, ensure it is fully cooked, drained of excess fat, and roughly chopped. Drain any excess fat from the skillet.

Mix Custard & Assemble

Whisk custard

In a large mixing bowl, whisk together the 8 eggs, whole milk, half-and-half (if using), Dijon mustard (if using), remaining salt and pepper, and smoked paprika until smooth and slightly frothy.

Combine filling

To the custard, add the cooked onion and peppers, diced ham, and 1 3/4 cups of the shredded cheddar (reserve 1/4 cup cheddar and all monterey jack for the top). Stir briefly to distribute ingredients evenly.

Transfer to pan

Pour the mixture into the prepared 6x13 pan, spreading it into an even layer. Sprinkle the reserved cheeses evenly over the top.

Bake

Place the pan in the preheated 350°F oven. Bake uncovered for 30–40 minutes, or until the center is set (a knife inserted in the center should come out mostly clean) and the top is lightly golden. If the top browns too quickly, tent loosely with foil for the final 10 minutes.

Rest & Serve

Remove the casserole from the oven and let rest for 8–10 minutes to finish setting. Sprinkle chopped chives or parsley over the top, then slice into 4–6 generous portions. Serve warm.