Melt butter in a small non-reactive saucepan over low heat. Stir in lemon juice, mustard, pepper and all but 1/2 tsp. chives. Cover and keep warm over lowest heat. Place asparagus in a steamer basket over boiling water. Cover saucepan and steam 5-7 minutes, or until asparagus is bright green and tender. Transfer asparagus to a serving platter. Drizzle with lemon sauce and sprinkle with remaining chives.