For the Chicken: In a large pot, place the chicken and pour over the water, and season with salt and pepper. Cook on medium-low heat, until the chicken is cooked through.
For the White Sauce: In a medium sauce pan, melt the butter and add the onions. Fry the onions on a medium high heat until they are just translucent, don't let them turn golden.
Slowly pour in the milk, while continuously stirring. Keep stirring until the sauce thickens up. If you don't continuously stir, lumps might form in the sauce.