Combine 1 tbsp oil with garlic and parsley in a dish and set aside.
Cut each leg at the joint to remove the thigh from the drumstick.
Season chicken legs with salt and pepper, then coat with flour, shaking off any excess. Warm remaining oil in a large frying pan over a medium heat. Sauté the chicken pieces, about 10 minutes each side, until the skin is evenly coloured. Reduce heat to low, cover and cook for 10 more minutes. Transfer chicken to a plate and keep warm.
For the sauce, keep the frying pan on a low heat and cook onion with a small pinch of salt. When onion begins to colour, pour in the wine. Raise heat to high and deglaze the pan, scraping the bottom with a wooden spoon until the brown bits dissolve and the wine has almost completely reduced.
Add the tomatoes and continue to cook, shaking the pan and stirring until the excess liquid evaporates, about 10 minutes. Add olives, stir through the garlic and parsley oil, and simmer for a couple of minutes.
Place chicken pieces into a warmed serving dish, pour over the sauce, scatter with basil leaves and serve.