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

Preheat grill or a cast iron skillet to high.

Divide the beef into 8 (4-ounce) burgers. Season on both sides with salt and pepper, to taste. Grill or cook in the skillet for 3 to 4 minutes on each side for medium-rare doneness. During last minutes of cooking add 2 slices of cheese to each burger, cover grill and let melt, approximately 1 minute. Place burger on bun and top with Horseradish Mustard, lettuce, Grilled Vidalia Onions, pickles and ketchup.

Horseradish Mustard:

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

Whisk mustard and horseradish together in a small bowl.

Grilled Vidalia Onions:

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

Brush olive oil on both sides of the onions and season with salt and pepper. Grill the onion slices for 3 to 4 minutes on each side until golden brown.