Combine the oat milk and water into a medium saucepan. Heat over medium-high heat and bring to a low boil. Add the oatmeal, oat bran, flaxseed meal, chia seeds, light brown sugar, sugar, salt, cinnamon, and cardamom to the saucepan. Mix to combine well.

Reduce the heat to low and simmer for 15 minutes. If the gruel thickens too quickly, add additional water to loosen the mixture to your liking. Split between two bowls or one bowl for an extra large serving. For an extra kick of flavor, top with your favorite berries.