Whisk together mustard, vinegar, lemon juice, sugar, salt, mayonnaise, and sour cream in a small bowl. Refrigerate dressing, covered, until ready to use, or up to 2 days.
Put cabbage, carrots, and onion (if desired) in a large bowl. Pour in dressing, and toss thoroughly. Refrigerate, covered, until slaw begins to soften, 1 to 2 hours. If not using immediately, refrigerate, covered, up to 2 days. Just before serving, toss coleslaw again.
