Follow the steps in order for the best result.
Begin by cooking the bratwursts thoroughly. Once cooked, drain any excess grease and cut the bratwursts into bite-sized chunks.
In a large bowl, combine the cooked bratwursts with the potatoes, chopped onion, drained green beans, cream of mushroom soup, milk, and cheddar cheese. Ensure all ingredients are well mixed.
Grease a casserole dish with butter to prevent sticking. Pour the bratwurst mixture into the prepared casserole dish.
Cover the casserole dish with a lid or aluminum foil and bake in a preheated oven at 350°F (175°C) for 45 minutes, or until heated through and bubbly.