Broil poblano and bell peppers on a foil-lined baking sheet on top rack of oven for 12 minutes, turning midway through cooking time or as needed. Peppers should be charred and blackened. Make a foil packet by folding sheet of foil in half. Place peppers into packet and fold edges to seal in peppers. Let peppers cool for 5-10 minutes. Peel and seed foil, taking care not to touch eyes. Nitrile cooking gloves are recommended. Remove to a cutting surface and slice peppers into 1/2-inch-wide strips.
Preheat oven to 400° F.
Toss onion, oil, pumpkin, potato, and garlic together. Sprinkle with flour, oregano, cumin, salt, and ground red pepper. Toss and coat thoroughly. Assemble casserole by alternating layers of pumpkin mixture and peppers in a 13- x 9-inch baking dish coated with cooking spray. Pour evaporated milk over vegetables. Spray casserole with cooking spray and over with foil. Bake at 400° F for 45 minutes. Uncover and sprinkle with cheese, baking 10 minutes or until potatoes and pumpkin are tender.