Step 1

Position a rack in the middle of the oven and preheat to 375 degrees.

Step 2

In a large skillet over medium-high heat, cook the beef and onions, breaking up the meat with a wooden spoon and browning the beef until it just starts to turn crispy, about 10 minutes. Using a slotted spoon, transfer the mixture to a medium bowl, leaving behind as much fat as possible in the pan. Season the mixture with the cumin, 3/4 teaspoon of the salt and the black pepper.

Step 3

Grease a 9-inch square baking dish with the butter or the fat rendered from the beef. Gently remove the stems and seeds from the poblanos and open the peppers so the flesh lies flat. Line the bottom of the pan with the poblanos, skin side down, overlapping them some to cover completely. Chop any remaining poblanos and set aside. Layer the meat-onion mixture over the whole poblanos and top with the shredded cheese and the reserved chopped peppers.

Step 4

In a medium bowl, whisk together the flour and the remaining 1/4 teaspoon of salt. While whisking, gradually pour in the milk, breaking up any clumps of flour. Whisk in the eggs and pour the mixture evenly into the baking dish.

Step 5

Bake the casserole for 45 minutes, or until it has puffed and browned. Remove from the oven and serve.