Place a 6-quart pressure cooker coated with cooking spray over medium-high heat until hot. Add onion, and saute 1 minute. Add rice, millet, and quinoa, and saute 30 seconds. Stir in water, apple juice, and cinnamon stick. Close lid securely, and bring to high pressure over medium-high heat (about 4 minutes). Adjust heat to low or level needed to maintain high pressure, and cook 5 minutes. Remove from heat, and place pressure cooker under cold running water. Let stand 7 minutes, and remove lid. Discard cinnamon stick; fluff mixture with a fork. Spoon mixture into a large bowl; cool 5 minutes.
Stir in the diced apple and remaining ingredients. Serve the salad chilled or at room temperature.