To keep things tidy, place a sheet of plastic wrap over the chicken breasts. Lightly pound them to an even 1/2-inch thickness (this will help them cook more quickly and evenly). Transfer the pounded chicken breasts to a ziptop bag or small baking dish that can hold them snugly.
In a large measuring cup with a spout or a bowl, whisk together the marinade ingredients: oil, garlic, lemon zest and juice (zest the lemon directly over the cup), thyme, oregano, salt, and pepper. Pour all but about 3 tablespoons over the chicken (reserve the rest for cooking the vegetables). Toss to coat the chicken evenly, then let the chicken marinade at room temperature for 30 minutes, or refrigerate for up to 2 hours (I don’t recommend marinating much longer as the lemon juice will start to break down the chicken). Let come to room temperature prior to cooking.
Roast the vegetables: Preheat the oven to 425°F. For easy clean up, line a large, rimmed baking sheet with parchment paper. Place the tomatoes and onion slices in the center and toss with the reserved marinade. Spread into a single layer and roast for 10 minutes.
Remove the pan from the oven, toss the tomatoes and onion slices, then scoot them towards the edges of the pan to clear a place for the chicken in the center. Place the chicken breasts in the space so they are not touching (it's OK for the tomatoes and onions to overlap). Return to the oven and continue roasting until the chicken is cooked through, about 15 to 20 minutes; it should register 155°F on an instant read thermometer inserted at the thickest part of the meat (its temperature will rise as it rests). Remove the chicken to a plate, cover, and let rest 5 minutes.
Spread the tomatoes and onions back into a single layer. Sprinkle the olives over the top. Return to the oven and continue roasting while the chicken rests to further crisp the onions and warm the olives (if the vegetables are already done to your liking, turn off the oven and return the pan to the oven during the last few minutes of the chicken’s rest time to warm them through). Place the chicken back on the pan with the vegetables. Sprinkle everything with the feta and fresh parsley. Enjoy hot.
Notes
TO STORE: Transfer leftovers to an airtight container and refrigerate it for 3 to 4 days.
TO FREEZE: Freeze Greek chicken in a freezer bag (this is your best option, because you can press the air out) or airtight container for up to 2 months.
TO REHEAT: If you’ve frozen the chicken and veggies, thaw them in the refrigerator first. Reheat them covered in a baking dish at 350ºF until warmed through, in a skillet over medium heat, or in the microwave if that’s your only option.