Preheat the oven to 375 degrees.Spray a 9 x 13 baking dish with cooking spray and set aside.

Cook the bratwurst in a large skillet over medium heat for 5-7 minutes or until browned. Remove from pan.

Add 1 tablespoon butter to the same skillet and melt over medium heat. Add in the onions and garlic and cook for 2-3 minutes.

Add the remaining butter and melt. Add in the flour and stir until it begins to brown.

Reduce heat to medium low and add in the milk, beer, and mustard. Stir until well combined. Cook for 5 minutes.

Add in the cheddar cheese, salt, and pepper. Stir over medium low heat until cheese is melted. Stir in the bratwurst and remove from heat.

Layer half of the potatoes int the pan and sprinkle with salt and pepper. Pour half of the cheese and beer mixture over top of the potatoes.

Layer with the remaining potatoes and then pour the remaining cheese and beer mixture over top of the potatoes.

Cover with foil and bake for 1 hour.

Remove foil and continue cooking for 30 minutes to an hour or until potatoes are cooked through.