Ingredients
1/4 cup mayonnaise
1/4 cup spicy mustard 
3 to 4 tablespoons poppy seeds 
3 tablespoons grated onion 
1 tablespoon horseradish mayonnaise
12 soft hamburger buns, sliced in half
1 pound thinly shaved roast beef 
8 ounces sliced provolone 
8 ounces American cheese