Combine the onion, garlic, and the rosemary in a saucepan along with enough water to cover the chicken breasts, and bring to a boil.
Simmer over low heat for 5 minutes, then add the chicken breasts.
Simmer for 5 minutes, then remove from the heat.
Allow to sit in the poaching liquid for 10 minutes, then place the pan in the refrigerator, complete with the poaching liquid.
Allow to chill for 2 to 3 hours.
For the sauce, combine the watercress, mayonnaise, sour cream, shallot, salt, and pepper in a food processor or electric blender and puree until smooth.
To serve, remove the breasts from the poaching liquid and drain on paper towels.
Place a dollop of the sauce on top of each breast, and put the rest of the sauce in a bowl for diners to serve themselves.