In a large bowl, combine the coleslaw mix, onions, 1/4 cup mayonnaise, vinegar, salt and pepper. Cover and refrigerate until chilled.
Meanwhile, combine mustard and remaining mayonnaise. Spread over one side of each slice of toast; top with a cheese slice, corned beef and another cheese slice. Place on foil-lined baking sheets.
Bake at 450° for 5-6 minutes or until cheese is melted. Top each with 1/4 cup coleslaw.
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