Soak ancho chilies in hot water for 15 minutes. Remove the stem and discard.
Place the chili in a food processor bowl with the onion, oil and salt. Process until a smooth paste forms.
Rub the ancho chili paste all over the chicken breasts. Place the chicken breast in a food safe bag and vacuum seal them. If desired, let the chicken marinate in the fridge for 1-2 hours before cooking them in the sous vide.
Place vacuum sealed chicken in the water bath an cook for about 2 hours.