How to Make It
Melt the butter in a large, deep skillet. Season the chicken with salt and pepper. Add half of the chicken to the skillet and cook over moderate heat until golden brown, about 4 minutes per side; transfer to a large plate. Repeat with the remaining chicken. Add the onions to the skillet and cook over moderate heat, stirring occasionally, until lightly browned, about 8 minutes.
Spoon off all but 1 tablespoon of the butter from the skillet. Return the chicken to the skillet, add the thyme and bay leaves, cover tightly and simmer over very low heat until the chicken breast pieces are just white throughout, about 40 minutes. Transfer the breast pieces to a large plate and cover to keep warm. Continue simmering the chicken in the skillet until cooked through, about 15 minutes longer.
Stir in the stock and return the chicken breast pieces to the skillet. Season the chicken sauté generously with salt and pepper and serve.