Hand-whisk together all marinade ingredients in a large bowl until well combined. Set aside.
Towel-dry chicken legs and use a fork to pierce holes throughout chicken pieces. Add to the large bowl with marinade. Turn to coat well and let sit in marinade for 1 hour at room temp.
Meanwhile, preheat oven to 425F, with rack on lower middle position. In a large baking dish, place chicken legs in a single layer. Evenly distribute marinade over the chicken. Arrange onion and lemon slices in between and around chicken pieces.
Bake uncovered for about 1 hour, basting with juices from the pan after 30 minutes. Baste every 10 minutes thereafter, until chicken is done and nicely browned. Let rest at room temp 5 minutes before serving. Serve chicken with juices poured over the top.