Preheat the oven to 425 degrees . Place the chicken and head of garlic in a large cast-iron skillet. Rub the chicken and garlic with 1 tbsp. oil; season with salt and pepper. Using kitchen twine, tie the legs together. Roast 25 minutes.
In a large bowl, toss the bread, onions and 2 tbsp. oil; season. Arrange the bread and onions around the chicken. Roast until an instant-read thermometer inserted into the thickest part of the thigh registers 165 degrees , about 35 minutes more. Transfer the chicken to a platter.
On a cutting board, finely chop the parsley, lemon zest and the remaining garlic clove to make the gremolata; season.
Sprinkle the chicken with half the gremolata. Using tongs, squeeze the cloves from the head of garlic into the skillet. Add the vinegar and the remaining gremolata to the skillet; toss to combine. Serve with the chicken.
