Heat pan used for chicken over medium-high heat. Add ¼ cup water (⅓ cup for 4 servings), a squeeze of lemon juice, stock concentrate, and remaining mustard. Simmer until slightly thickened, 1-2 minutes. Turn off heat. Stir in sour cream and remaining chives. (TIP: If sauce seems too thick, add more water, 1 tsp at a time, until it reaches drizzling consistency.) Season with salt and pepper.