Preheat the oven to 500 degrees F.
Heat the olive oil in a heavy sauté pan over medium-high heat. While the oil is heating, season the chicken breasts with salt and pepper. Add the chicken and garlic to the pan, skin side down. Cook the chicken for about 5 to 7 minutes or until the skin is crisped and golden.
Then, remove the chicken and garlic from the pan. Reduce the heat to medium, and add the lemon slices, onions and thyme to the pan. Cook the onion and lemons, stirring occasionally, until soft and caramelized. Season with salt and pepper. Remove the sprigs of thyme, and sprinkle the flour over the pan, stirring to cook for about a minute. Slowly add the wine stirring allow to come to a boil for a minute. Add the chicken stock, and the chicken back into the pan. Cook to reduce the sauce and warm the chicken through. Stir in the unsalted butter, and place the entire pan in the oven for 20 minutes. Remove and serve.