Spring-Vegetable Couscous with Chicken

In a medium saucepan, melt butter over medium-high. Add scallion whites and cook, stirring constantly, until softened, about 3 minutes. Add lemon zest and 1 1/4 cups water and season with salt and pepper. Cover pan and bring liquid to a boil, then add asparagus and peas. Return to a boil and stir in couscous. Add chicken and remove pan from heat; cover and let sit 7 minutes. Add lemon juice and parsley and season with salt and pepper. Fluff couscous with a fork and serve.