Bring water to boil in a small pot. Add rinsed buckwheat and millet, plus oats, apple, almonds, raisins or dates, cinnamon, cardamom, and salt. Stir. When simmering, cover, reduce heat to low, and cook 15 minutes. Turn off heat and let pot sit, covered, for 5 minutes. Stir in flaxseed and vanilla. Serve with milk or yogurt, if desired.