In large skillet with lid, brown ground meat and onion. Drain grease, return to pan and put on medium heat. Add peas with liquid. Slowly sprinkle and stir in flour, making sure to get all lumps. Bring to a low boil, remove from heat. Smooth out meat/peas mixture, top evenly with mashed potatoes. Sprinkle with cheese, cover pan so that the cheese melts.