Preheat the oven to 400 degrees F. Cut the top third off the head of garlic to expose the cloves. Place on a piece of foil and drizzle with oil and sprinkle with salt. Wrap the head in the foil and roast until the garlic is very tender, about 30 minutes.
Meanwhile, combine the bread and milk in a small bowl and let soak until the milk is completely absorbed, about 10 minutes.
Line a rimmed baking sheet with foil. Combine the bacon, onion and minced garlic in the bowl of a food processor and pulse until finely chopped. Transfer to a medium bowl along with the ground beef, soaked bread, 2 tablespoons of the chives, 1/2 teaspoon salt and a few grinds of pepper. Combine gently with your hands and form into 24 meatballs, about 1 1/2-inches in diameter (dampen your hands with water to make rolling easier). Arrange the meatballs on the prepared baking sheet and bake until cooked through, about 20 minutes. Turn the oven to broil and cook until the meatballs are browned, 3 to 5 minutes.
Meanwhile, squeeze the roasted garlic out of the white paper husks, place it in a small bowl and mash with a fork. Season it with 1/4 teaspoon salt and a few grinds of black pepper. Stir in the creme fraiche, mustard, and the remaining 2 tablespoons chives.
Build the sliders by placing a small piece of lettuce on the bottom of a bun, top with a meatball, spread the top bun with the roasted garlic cream and place on top. Secure with a toothpick or short bamboo skewer. Repeat with remaining ingredients. Serve warm.