Preheat a grill to medium high. Tear off a 20-inch-long piece of heavy-duty foil. Put the onion, garlic and bay leaf on the foil, piling them on one side; season with 1/4 teaspoon salt and a few grinds of pepper. Dot with the butter and place the bratwurst on top. Fold the foil over the onions and bratwurst, then fold in the edges on two sides to seal, leaving one side open. Carefully pour the beer into the open side, then fold in the edge to seal.
Place the foil packet on the grill onion-side down; cook 10 minutes, then carefully flip and continue grilling until the onion is softened and the bratwurst is warmed through, 10 more minutes. Meanwhile, grill the rolls until toasted.
Carefully open the packet. Remove the bratwurst and grill, turning, until lightly charred all over, 3 to 5 minutes. Return to the packet to toss with the onion. Serve with the rolls, sauerkraut and mustard.