In a medium bowl, combine chicken, 1 tbsp dry sherry, 1 tsp cornstarch, 1/2 tsp salt and pepper.
In a small bowl, combine broth, soy sauce, remaining 2 tbsp dry sherry and 1 tsp cornstarch.
Heat a 14-inch flat bottomed wok or 12-inch skillet over high heat until a drop of water evaporates within 1 to 2 seconds of contact; swirl in 3 teaspoons oil. Add shallots and garlic; stir-fry 10 seconds or until fragrant. Push shallot mixture to side of wok/skillet and add chicken; spread evenly in one layer in wok/skillet. Cook undisturbed 1 minute, letting chicken begin to sear. Stir-fry 1 minute until chicken is no longer pink, but not cooked through.
Swirl in remaining 1 tsp oil. Add green beans and capers and sprinkle on remaining 1/4 tsp salt; stir-fry 30 seconds or until just combines. Swirl in broth mixture; stir-fry 1-2 minute or until chicken is cooke through and sauce has lightly thickened.