Remove giblets and rinse chicken well under cold water.
Place in large metal roasting pan and pat dry with paper towels.
Season with Kosher salt and black pepper.
In a separate bowl, mix olive oil, rosemary, Herbes de Provence, the juice from both lemons, and garlic.
Rub cut side of squeezed lemon halves over chicken and insert into cavity.
Peel and quarter onion and insert that into the cavity as well.
Pour olive oil mixture over chicken.
If also making potatoes, wash, dice, and arrange around chicken.
Add peeled garlic cloves to potatoes and drizzle with a little more olive oil, then season with pepper and more Herbes de Provence.
Bake at 425 degrees, basting occasionally and stirring potatoes, about 1.5 hours or until meat thermometer reads 165 degrees.
Remove from oven and tent with foil; let rest covered for about 10 minutes, then remove and discard lemon halves and onion before carving.
Notes
If you're making a smaller chicken, adjust the seasonings, oil, and cooking time proportionately and cook until it reaches an internal temperature of 165 degrees. (Even if your chicken has one of the little pop-up things in it, don't rely on that to test done-ness. A meat thermometer is much more reliable.) Be sure to let it rest before carving to help preserve the juiciness, as tempting as it will be to dig into your chicken right away. If your potatoes need a little extra time, just pop them back in the oven while your chicken is resting.