In a small saucepan steam broccoli until tender. Drain and puree in blender with lime juice. Heat stock to boiling. If using agar, cook in stock over medium-high heat until dissolved. If using gelatin, dissolve in hot stock. Let cool. Mix with broccoli. Add green onions, egg white, mayonnaise, and dill to broccoli mixture. Lightly oil a 1-1/2-quart soufflé dish and pour mixture into it, then chill in refrigerator for 3 hours. While mousse is chilling, mix yogurt, beets, and horseradish, and chill for 1 hour. Stir well again and set aside until serving time. To serve, spoon equal amounts of broccoli mousse onto 4 salad plates lined with lettuce leaves. Garnish each with a dollop of horseradish sauce.