Combine all ingredients for the marinade and season with salt and pepper.
Rinse the chicken, pat it dry, and place it in the marinade. Cover the bowl and refrigerate for an hour.
Wash and scrub the potatoes. Peel and chop the red onion. Halve the head of garlic lengthwise. Place everything on a baking sheet, drizzle with some oil, and season with salt and pepper.
Remove the chicken from the marinade and place it on the baking sheet. Brush the chicken with the remaining marinade and place the rosemary sprigs on the vegetables.
Bake for 50 minutes, until the chicken is tender and the potatoes are cooked through.