Preheat your oven to 375°F. While it heats, prepare your mise en place by dicing the red onion and red bell pepper into 1/2-inch pieces, dicing the green pepper to the same size, and cutting the ham into bite-sized pieces. This prep work ensures everything is ready before you start cooking. I like to use room temperature eggs here—they'll incorporate more smoothly into the custard and give you a better rise.
Melt the butter in a 10-inch cast iron or oven-safe skillet over medium heat. Add the diced onions and peppers, cooking for about 5 minutes until they begin to soften and release their aromatics—you want them slightly tender but still with a bit of texture. Push the vegetables to the side of the skillet, then add the ham pieces and cook for 2-3 minutes until they're slightly caramelized and flavorful. The vegetables will continue cooking in the oven, so don't worry about them being fully tender.
In a bowl, crack the eggs and whisk them together with the milk until well combined and slightly frothy. Add the garlic powder, onion powder, paprika, parsley, black pepper, salt, and red pepper flakes, whisking until all spices are evenly distributed. I prefer to use Cabot extra sharp white cheddar for this recipe—it has more punch than mild cheddar and adds real flavor to every bite. Measure out 1/3 cup of the cheese and stir it into the egg mixture, reserving the remaining cheese for topping.
Pour the egg custard mixture from Step 3 over the sautéed vegetables and ham in the skillet, distributing everything evenly. Sprinkle the reserved cheese over the top so it gets golden and melty during baking. Transfer the skillet to the preheated 375°F oven and bake for 15-18 minutes, until the eggs are set around the edges but still slightly jiggly in the very center—the residual heat will continue cooking it as it cools.
Remove the frittata from the oven and let it rest for 2-3 minutes before slicing. This brief rest allows the proteins to set fully and keeps the frittata from falling apart when you cut into it. Slice into wedges and serve warm directly from the skillet for a rustic presentation.