Whisk together the ketchup, mayonnaise, vinegar, celery seeds, paprika, and some salt and pepper.
Put the cabbage, carrot, green onions and Spanish onions in a large bowl; season with salt and pepper, and toss to combine. Pour the dressing over the vegetables and toss again. Cover and refrigerate for at least 30 minutes and up to 8 hours.
Recipe courtesy of Bobby Flay