2 When you are ready to cook, put the chicken legs into a roasting dish with the marinade, onions and the tomatoes. Cook in the oven for 15-20 minutes until the chicken is cooked through. The tomatoes should burst and the onions will go crispy around the edges. Meanwhile, cook the potatoes in the microwave at 800W for eight minutes - you can pop them in the oven with the chicken when they are done to crisp up the skins.