Preheat oven to high broil. Wash peppers, pat completely dry, and place them on a baking sheet. Broil for several minutes, turning halfway, until very blackened on both sides.
Place chiles in a plastic bag and tie the bag, or place in a bowl and cover with plastic wrap to allow them to steam. Set aside for 5-10 minutes until they are cool enough to handle.
Open the bag and gently peel the charred outer skin off of the chiles—it should come off easily.
Starting at the top stem of the chiles, use a knife to make a small slit into the side of each chili about 1.5 inches long. Use your finger or a small spoon to gently remove the seeds and membrane, (being careful not to tear the peppers). Cut the cheese into chunks and carefully stuff a few pieces in to fill each chili.
Heat the oil in a large skillet over medium heat.
Prepare the batter. Separate the egg whites into a small mixing bowl, and the egg yolks into a large, deep mixing bowl. Whip the egg whites with an electric mixer until stiff peaks. Set aside.
To the bowl with the egg yolks, add milk, 1/2 cup flour, baking powder, baking soda, and salt. Gently fold in the beaten egg whites.
Coat and Fry:
Add ¼ cup of flour to a shallow bowl or plate and gently coat chiles in flour on both sides—they only need a light coating, which will help the batter adhere to them.
Check the oil to make sure it’s ready by adding a small drop of batter to it. It should gently sizzle and begin to cook and turn golden.
Lay one of the chiles into the batter and gently spoon batter over it to cover it in a thin layer. Pick the chili up carefully by the stem and lift from the batter, allowing any excess to drip off, and then carefully lay it in the hot oil. Cook for 1-2 minutes, until golden brown on the bottom, and then flip to the other side.
Transfer to a wire cooling rack lined with paper towels. Serve warm, topped with salsa roja.
Check the oil temperature: Check the oil to make sure it’s ready by adding a small drop of batter to it. It should gently sizzle and begin to cook and turn golden. If you have a thermometer, it should register about 375F.
Cook in small batches: Cook just a few chiles at a time (2-3) depending on the size of your pan and burner. If you try to cook too many, it will decrease the temperature of the oil and the chiles will soak up excess amounts of oil rather than frying perfectly.
Stuff with meat: Substitute cheese for shredded or ground pork or beef.
Stuff with veggies: Any assortment will work.
Spicy chiles relleno: stuff with cooked bacon, jalapeños, and cheese.
Chile relleno casserole: Broil the peppers and make salsa roja. Fry eggs in a frying pan in one flat layer. In a casserole dish spoon a thin layer or salsa, add eggs, 2-3 sliced poblano peppers, more sauce, and cheese. Repeat with another layer. Bake at 375 degrees for 30-45 minutes and allow to cool for 15 minutes before eating.
Make-ahead Instructions:The polano chiles can be roasted and stuffed with cheese several days in advance, stored covered in the fridge. The salsa verde can also be made several days in advance.