Hamburger with Double Cheddar Cheese, Grilled Vidalia Onion and Horseradish Mustard

• 2 pounds freshly ground chuck
• Salt and freshly ground black pepper
• 8 slices white Cheddar, sliced 1/4-inch thick
• 8 slices yellow Cheddar, sliced 1/4-inch thick
• 8 hamburger buns
• Horseradish Mustard, recipe follows
• 8 leaves Romaine Lettuce
• Grilled Vidalia Onions, recipe follows
• Dill pickles, sliced
• Ketchup

Horseradish Mustard:

• 1/2 cup Dijon mustard
• 2 tablespoons prepared horseradish, drained

Grilled Vidalia Onions:

• 2 Vidalia onions, sliced crosswise, 1/4-inch thick slices
• 2 tablespoons olive oil
• Salt and freshly ground black pepper