1 pound boneless, skinless chicken thighs, cut into 1/2-inch pieces
1 tablespoon plus 2 teaspoons cornstarch
2/3 cup low-sodium chicken stock
1 tablespoon rice vinegar
1 tablespoon brown sugar
1/2 teaspoon garlic chili sauce
1 tablespoon plus 2 teaspoons soy sauce
2 tablespoons vegetable oil, such as safflower
1 leek, halved lengthwise, well-washed and thinly sliced crosswise
1/2 medium jicama (about 6 ounces), peeled, thinly sliced, and cut into matchsticks
1 red bell pepper, ribs and seeds removed, cut into 1/2-inch strips
2 tablespoons minced peeled ginger
3 garlic cloves, minced
3 bunches watercress (about 21 ounces), thick stems trimmed, coarsely chopped