Leftover suggestion: Make a quesadilla with added cheese and fresh toppings such as pico de gallo and guacamole.
Preheat oven broiler to high and prepare a baking sheet with foil. Place the whole peppers on the baking sheet, on the oven rack set closest to the oven element. Roast for 7-10 minutes until the pepper skin starts to become charred and black with some green color remaining. Turn with tongs to roast other side for about 7-10 minutes. Remove from the oven to cool slightly and turn the oven temperature to 350ºF.
In a medium-sized skillet, over medium-high heat, add oil, onions, and salt. Sauté for 5 minutes, until translucent.
Add garlic and cook for 2-3 minute, until fragrant. Remove from heat and reserve to the side.
In a medium-sized mixing bowl, whisk together milk, eggs, and cumin, and reserve to the side.
Once the peppers are cool to the touch, peel as much of the black skin away as possible, and discard. Chop peppers and distribute half into the bottom on a small casserole dish.
Pour strained salsa and enchilada sauce over the peppers, layer with 1/2 the cooked onions and garlic, black beans, feta cheese, the remaining cooked onions, garlic, and peppers.
Pour the egg mixture over the casserole and sprinkle cheese over the top. Bake for about 45 minutes, or until the milk and egg mixture is set and no longer liquid.