Lightly pound the chicken breasts into an even thickness (place a sheet of plastic wrap over the top to keep it tidy). Transfer to a sturdy ziptop bag.
In a bowl or a large liquid measuring cup with a spout. Add the olive oil, garlic, rosemary, salt, and pepper. Zest both lemons into the bowl, then juice in 1 of the lemons. Whisk to combine, then pour over the chicken. Reserve the second lemon.
Seal the bag, removing as much air as possible, turn to coat the chicken, then place in a dish or on a plate to catch any drips. Marinate in the refrigerator for 1 hour or up to 8 hours.
When ready to cook, remove the chicken from the refrigerator and let stand at room temperature while you preheat the oven to 425°F. Cut the remaining lemon into wedges. Line a 9x13-inch baking dish with parchment paper.
Remove the chicken from the marinade, shaking off any excess. Arrange the chicken in a single layer in the dish. Scatter the lemon wedges around the chicken.
Bake the lemon chicken for 14 to 16 minutes (for small/medium breasts that are about 6 to 7 ounces), 16 to 20 minutes (for medium/large breasts that are 8 to 10 ounces), or 20 to 25 minutes (for larger breasts). For absolute best results, use an instant read thermometer. When the breasts reach 165°F, they are done (I prefer to remove chicken at 155°F, as its temperature will rise as it rests).
Place the chicken on a cutting board. Cover and let rest for at least 5 minutes. Serve with the baked lemon wedges
Notes
TO STORE: Refrigerate chicken breast in an airtight storage container for up to 3 days.
TO REHEAT: Gently rewarm chicken in a baking dish in the oven at 350°F, or cut it into pieces and gently microwave (splash some water or broth over the chicken to keep it moist).
TO FREEZE: Freeze leftovers in an airtight freezer-safe storage container for up to 3 months. Let thaw overnight in the refrigerator before reheating.