I wanted to make a chile relleno casserole for a potluck, but didn't love any the recipes I saw. So I combined several recipes and made my own version. It has lots of green chiles and both jack and longhorn cheese, for maximum Tex-Mex appeal. The final dish is great to take for a potluck dinner, but can also be served for breakfast and brunch. It's best just out of the oven when the egg batter is fluffy, but it is still good reheated or even served at room temperature.
A few notes about the ingredients. You should try to use the best canned green chiles you can find because they give the casserole most of its flavor. The whole canned ones from a name brand (e.g. Hatch) work well. Store-brand diced green chiles will produce a less flavorful result. Home-roasted and peeled fresh green chiles (like a poblano) would be excellent, but I don't usually have the patience for that. Also, the tomato/chile sauce I'm referring to for topping the casserole is a Mexican-American brand, El Pato. It comes in small (8 oz) yellow cans and has a duck (pato?) on the can. It's quite spicy and is the closest canned product I can find to the relleno sauces I'm used to, which are more tomato-y than a standard enchilada sauce. Leftover El Pato is great on eggs or tortillas or just about any Mexican dish. In a pinch, though, you could use red chile enchilada sauce or blend up some Ro-tel until it's not chunky. Don't substitute plain tomato sauce, it doesn't have enough flavor.