In a microwave-safe glass pie plate coat the chicken with 2 teaspoons of the oil, arrange it with the thickest parts toward the edge of the plate, and pour the broth over it. Microwave the chicken, its surface covered with an oiled round of wax paper, at high power (100%) for 4 minutes. Turn the chicken, microwave it, covered with the wax paper, at high power for 3 minutes, and microwave it at medium power (50%) for 3 minutes more, or until it is just springy to the touch. Let the chicken stand for 5 minutes.

While the chicken is standing, in a small bowl whisk together the mustard, the lemon juice, the tarragon, the scallion, and salt to taste and whisk in 3 tablespoons of the cooking liquid and the remaining 1 tablespoon oil. Transfer the chicken to a shallow dish, spoon the vinaigrette over it, and let the chicken cool. Serve the chicken at room temperature.

Serves 2.