Brown the shallots in the same skillet. Add oil if necessary. Season with salt and pepper. Add the prunes and garlic and cook for 1 minute more. Add the wine. Bring to a boil and reduce until the mixture starts to become syrupy. Add the chicken and the thyme. Bake until the chicken is cooked through, about 10 minutes. Remove the chicken. Add the butter to the sauce and stir until melted. Adjust the seasoning.