Preheat the oven to 350 degrees F.
Combine the vinegar, oil, salt, pepper, herbes de Provence, and lemon juice in a bowl, whisking until emulsified.
Pour over the chicken, taking care to completely coat it. Insert the reserved squeezed lemons, plus the rosemary and garlic into the cavity. Layer the onions in a roasting pan and place the chicken on top. Slice the fennel in half lengthwise and surround the chicken with it, cut side down.
Put the chicken in the oven and immediately reduce the temperature to 250 degrees F and roast for 1 1/2 to 2 hours, basting occasionally.
Recipe courtesy of Reggie Southerland