Preheat the oven to 275ºF. Squeeze about 3 Tbsp lemon juice from the lemon(s) and slice the remainder. Mince two cloves of the garlic, and peel the remaining 4. In a small bowl, whisk together the lemon juice, olive oil, minced garlic, salt, and pepper.
Pat the chicken breasts dry with a paper towel. Place them in a glass or ceramic casserole dish. Pour the prepared lemon marinade over the chicken, using a spoon to redistribute the marinade and make sure the surface of the chicken is fully covered. Add the remaining cloves of peeled garlic and sliced lemon around the chicken. If you’re sensitive to bitter flavors, add the additional lemon slices after baking. Cover the dish tightly with foil.
Roast the chicken in the preheated oven for 90 minutes. After 90 minutes, turn the oven up to 425ºF, remove the foil, and baste the chicken with the juices from the bottom of the dish. Return the chicken to the oven and roast for an additional 20 minutes without foil at 425ºF, or until the skin is golden brown and crispy.
Let the chicken rest for 5-10 minutes before slicing. Serve warm with the pan drippings spooned over top.