Prep: Preheat the oven to 350F.
Dry Ingredients: Add the rolled oats, quick oats, flax seed, cinnamon, cardamom, baking powder, and salt to a 9×13” baking dish and mix well.
Wet Ingredients: Add the milk, maple syrup, lemon juice, vanilla, and thawed blueberries and mix again. Top the oatmeal with additional fresh blueberries, if desired.
Bake: Bake for 30 to 35 minutes, until the top of the oatmeal is golden (Note: if you want to serve a runnier oatmeal, bake for only 20 to 25 minutes).
Serve & Store: Serve immediately for a softer oatmeal, or let the oatmeal cool in the baking dish, then slice and serve into bars. Serve as-is or with a drizzle of nut butter and/or milk. Store leftovers in the refrigerator for up to 5 days, or pre-portion the oatmeal and store in the freezer for up to 2 months.