Rinse the millet in a fine-mesh strainer under cold running water for about 30 seconds, until the water runs mostly clear. Drain well.
Add the rinsed millet to a dry 2-quart / 1.9 L saucepan and toast over medium heat for 3 to 4 minutes, stirring often, until it smells nutty and looks lightly golden.
Pour in the water, oat milk, salt, and cinnamon. Bring to a boil, then reduce the heat to low.
Cover and simmer for 18 to 20 minutes, stirring only two or three times, until the millet is tender and most of the liquid is absorbed. It should look thick and loose, like cooked oatmeal, not soupy or gluey.
Remove the pan from the heat and stir in the vanilla and maple syrup. Cover and let it sit for 5 minutes, it will thicken more as it rests.
Spoon the porridge into bowls. Top each with banana slices, berries, chia seeds, and chopped almonds.
Drizzle with extra maple syrup and a splash of oat milk if you want it looser, then serve warm.