The Roast Beast Master with Crispy Onions and Creamy Horseradish Sauce

Name: The Roast Beast Master with Crispy Onions and Creamy Horseradish Sauce
Ingredients: 1/2 cup mayonnaise; 1/2 cup sour cream; 1/4 cup prepared horseradish; 2 teaspoons honey; 2 teaspoons Dijon mustard; Salt and freshly cracked black pepper; 2 cups buttermilk; 2 onions, thinly sliced root to tip; Vegetable oil, for frying; 2 cups all-purpose flour; 2 teaspoons cayenne; 2 teaspoons paprika; Salt and freshly ground black pepper; 8 fresh onion rolls, buttered and toasted until golden; 4 thick slices sharp Cheddar; 2 pounds good-quality rare roast beef, sliced thin