The New York–style onions can be made up to 3 days in advance and refrigerated in an airtight container. Before serving, reheat the onions in the microwave or in a saucepan over low heat until warmed through. You can also cook the hot dogs on a grill. Preheat the grill to medium; place the dogs on the grill and cook, turning them every minute or two, until the hot dogs have dark grill marks and are heated through, 7 to 8 minutes.