Place a small rimmed sheet pan into the oven and preheat to 425°F.
Meanwhile, pat the chicken with paper towels to remove any excess moisture. Drizzle 2 tablespoons of the oil over the chicken and rub to coat (you just want it to have a nice coat, not be drenched in it). Season generously with salt and pepper.
Carefully place the chicken, skin-side up, onto the hot sheet pan and roast for 30 minutes.
In a large bowl, combine the onion, tomatoes, capers, olives, remaining oil, 1/2 teaspoon salt and a generous pinch of pepper and mix well.
Scatter the tomato mixture around the chicken and roast for another 30 minutes. Halfway through roasting, turn the chicken and spoon the sauce over the legs. If sauce seems it’s getting too dry, add a little bit of water to the pan and stir together with the sauce (just start with 1-2 tablespoons, so you’re not making sauce too watery).
Bake until the chicken is cooked through and golden brown.
To stay low-carb, serve with cauliflower rice, zucchini noodles.