Bring 3 cups of water to a boil in a medium pot.
Stir in the oats and simmer, uncovered, over a medium-low heat for 10 minutes.
Stir in the quinoa and continue to cook for 10 minutes more.
Add the milk and cook, stirring occasionally, for another 5 minutes.
Stir in the chia seeds and vanilla, then remove from the heat, cover and set aside as you prepare the bananas.
The cereal will thicken as it sits.
Peel and slice the bananas diagonally into ½-inch thick slices.
Heat the butter in a large non-stick or cast iron skillet over a medium-high heat.
Add the bananas to the skillet and cook, turning once, until browned, about 1 minute per side.
Sprinkle with the brown sugar and the cinnamon and cook, stirring gently, until the sugar melts, about 1 minute.
Add a tablespoon or two of water if the pan seems dry.
Spoon the grain mixture into serving bowls, top with the bananas and walnuts, if using.
Drizzle with additional milk and/or sprinkle with additional sugar if desired.