In a mixing bowl, combine the chuck, sirloin, garlic, onion, 1 teaspoon of the salt, and 1/2 teaspoon of the pepper and mix gently but thoroughly to combine. Using a 2-ounce ice cream scoop, divide the mixture into 12 scant 2-ounce balls of meat. Using your hands, shape to form small patties about 3 inches wide and 3/8-inch thick. Place on a plate and cover with plastic wrap. Allow to sit, refrigerated, at least 1 hour and up to overnight for flavors to mingle.

In a small bowl whisk together the mayonnaise, sour cream and horseradish and season with the remaining 1/4 teaspoon of salt and 1/8 teaspoon pepper. Place in a nonreactive bowl and refrigerate until ready to serve, up to 3 days in advance.

When you are ready to cook the burgers, preheat a grill to high and lightly butter the cut sides of the buns with the melted butter. Wrap the buns in aluminum foil and place on the coolest part of the grill to warm while you grill the burgers.

Grill the burgers about 2 minutes per side for medium. Transfer to the warmed buns and garnish with pieces of the crispy bacon. Spoon a dollop of the horseradish mayonnaise on top of the bacon, then place the top portion of the bun over all. Serve the sliders hot.