In a large skillet, brown bratwurst in batches; drain. In a 5-qt. slow cooker, combine beer, sliced onion and sauerkraut; add bratwurst. Cook, covered, on low 7-9 hours or until sausages are cooked through., Preheat oven to 350°. In a small bowl, mix mayonnaise, chili sauce, ketchup, chopped onion, relish, garlic and pepper until blended. Spread over cut sides of buns; top with cheese, bratwurst and sauerkraut mixture. Place on an ungreased baking sheet. Bake 8-10 minutes or until cheese is melted.