Preheat the oven to 425 degrees F. Heat the olive oil in a large ovenproof skillet over medium-high heat. Season the chicken with salt and pepper and cook, skin-side down, until browned, about 5 minutes.
Separate the heads of garlic into cloves but do not peel. Flip the chicken; add the garlic and rosemary to the skillet and transfer to the oven. Roast until the chicken is cooked through but still moist, 15 to 20 more minutes.
Place the bread on a platter and top each slice with a chicken breast. Add the vinegar to the skillet and scrape up any browned bits with a wooden spoon. Add 3 tablespoons water and simmer until the sauce thickens slightly, about 2 minutes. Pour the sauce and garlic over the chicken and bread.
Tip: You can squeeze the garlic out of its skin and spread on the chicken or bread.
Photograph by Antonis Achilleos