Prep the chiles Roast, peel, and seed your Hatch chiles. Lay them flat on paper towels to dry a bit.
Layer the casseroleGrease a 9x13baking dish.Spread half the chiles in a layer on the bottom.Sprinkle with half the cheese (and half the meat if using).Add the rest of the chiles, then the rest of the cheese (and meat).
Make the egg mixtureIn a bowl, whisk eggs, milk, flour, baking powder, cumin, salt, and pepper.Pour evenly over the chile/cheese layers.
BakeBake uncovered at 375°F for 35–40 minutes, until puffed, set, and golden.
ServeLet cool for 10 minutes before slicing. Top with warm enchilada sauce or salsa, plus sour cream or avocado slices.