Preheat oven to 425 degrees F. Position rack in lower third of the oven.
Make the herbed butter: in a bowl, combine the softened unsalted butter, minced garlic, minced rosemary, herbs de provence, and lemon zest. Set aside.
Pat the chicken dry. Rub the chicken all over with the herbed butter, including under the skin (carefully separate the skin from the flesh to create "pockets" for the butter). Season the chicken generously with salt and pepper, including the insides of the cavity.
Stuff the cavity with the lemon, onion, and the rosemary sprigs...as many as you can fit inside. Tie chicken legs using kitchen twine. Tuck in wings under the chicken's back, or clip the wing tips to prevent them from burning.
Place the prepared chicken in a roasting pan, breast side up. Place the remaining lemons and onions on the pan. If using a pan with a roasting rack, add about 1-2 cups of chicken broth over the pan, about 1/4 inch deep. This will prevent the drippings from burning.
Roast the chicken for about 1 hour and 20 minutes, basting with the drippings halfway through. If needed, rotate the pan if you notice certain spots browning more than others - some ovens have hot spots. The chicken is ready when the a meat thermometer inserted in the thickest part of the thigh registers 165 degrees F. Note: roast for 20 minutes per pound.
Remove from oven. Tent with foil, and let rest for 15-20 minutes. Carve the chicken and serve. Enjoy!