Preheat the grill to medium heat. Place the bratwurst sausages on the grill and cook for about 15-20 minutes, turning occasionally until they are browned and cooked through.

In a skillet, melt the butter over medium heat. Add the sliced onions and sauté until they are soft and caramelized, about 10-15 minutes.

Once the bratwurst are cooked, place each sausage in a bun, top with sautéed onions, and serve with mustard.