For the stuffing:
Sauté onion in oil and butter over low heat. Add spinach and cook for about 10 minutes. Transfer to a bowl and add the ricotta, Parmesan and basil leaves.
Loosen the skin of each breast by slipping the finger between the flesh and the skin. Divide the stuffing among the breasts and neatly fill the pockets. Place the breasts skin side up in a baking dish. Sprinkle with olive oil and rub with thyme and salt.
Bake for 45 minutes at 375 degrees or until skin is brown. Cool chicken then cut in half.
Serve with the yogurt sauce. To make the sauce: Combine the yogurt, vinegar, tomato and basil. Season to taste with salt and pepper.