In medium mixing bowl, whisk together mayonnaise, mustard, brown sugar, vinegar and celery salt. Add cabbage and carrots. Toss to coat. Salt and pepper to taste. Cover and chill until ready to use.
Spread spicy mustard on one side of 4 slices of bread. Top with roast beef and coleslaw.
Place remaining 4 slices of bread over coleslaw. Press together with your hands and serve.
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