Poach the chicken: Melt the butter over medium heat in a wide, shallow Dutch oven or sauté pan. Sauté the onion and garlic in the butter, then add the white wine, if using, and the chicken breasts. Add enough water to cover the chicken breasts by 1 inch. Bring to a boil. Cover and reduce the heat to a bare simmer. Cook for 8 to 10 minutes or until the chicken is cooked through. Cool thoroughly.