For the Burgers: 

In a large bowl, using your hands, gently mix the chuck, sirloin, garlic, onion, salt and pepper.

Divide the mixture into 12 portions using a ¼ cup measure. Using your hands, shape each portion into a small patty, about 3 inches wide and 3/8-inch thick.

Place the patties on a platter or in a 9-by-13-inch baking dish, cover them with plastic wrap, and refrigerate them (for at least 1 hour or up to overnight) to let the garlic mellow and all of the flavors mingle.

For the Horseradish Mayonnaise: 

In a small bowl, whisk together the mayonnaise, sour cream and horseradish. Season the mixture with ¼ teaspoon of salt and 1/8 teaspoon of pepper. Cover the mixture, and refrigerate it until ready to serve.

Just before you grill your burgers, fry the bacon in a skillet over medium heat until crisp. (Or, preheat your oven to 400°F, place the bacon on a baking sheet lined with aluminum foil, and bake for 15 to 20 minutes.) Drain the bacon on paper towels, and break it into 2-inch pieces. Set aside.

To Cook the Burgers: 

Preheat your grill to high, and grill the burgers for about 2 minutes per side for medium. (If you’re using a grill pan, preheat it to medium, grill the burgers for about 5 minutes per side, and check them for doneness.)

While the burgers are on the grill, brush the insides of the rolls with the melted butter. Wrap the buns in aluminum foil, and place them on the coolest part of the grill to warm. (If you’re using a grill pan, preheat your oven to 250°F, place the open rolls on a baking sheet, brush them with melted butter, and let them toast in the oven while you grill the burgers.)

To Assemble the Burgers: 

Place a little lettuce on the bottom portions of the warmed rolls, and add the burgers on top of the lettuce. Top the burgers with bacon. Spoon a dollop of mayonnaise over the bacon, and place the bun tops over all. Dig in!