Combine almond milk, rosemary, and salt in a saucepan over medium-high heat; bring to a boil. Stir in oats, banana, and flaxseed. Reduce heat to medium and cook until most of the liquid has been absorbed, 4 to 5 minutes.
Add blueberries and lemon juice to the pan. Continue to cook until desired consistency is reached, 1 to 2 minutes more.