Bring 3 cups water, 1 cup milk, the cinnamon stick, and the salt to a boil in a medium saucepan. Stir in oats. Return to a boil. Reduce heat; partially cover. Cook, stirring occasionally, until mixture is thick and oats are tender, about 25 minutes. Discard cinnamon stick.
Cut banana crosswise into 1/4-inch-thick rounds. Top porridge with banana and dates. Sprinkle with flaxseed. Drizzle each serving with 1 tablespoon milk.