Rinse the amaranth in a fine-mesh sieve under cold water for 30 seconds, then shake off the excess. Roughly chop the walnuts.
Add the amaranth, water, milk, cinnamon and salt to a 3-quart / 2.8 L saucepan. Bring to a boil over medium-high heat, about 5 minutes, stirring once or twice.
Reduce the heat to low, set the lid slightly ajar, and simmer for 20 minutes. Stir every 4 to 5 minutes and scrape the bottom, because amaranth sticks and scorches fast. It's ready when the porridge is as thick as loose oatmeal and the grains are soft with a slight pop.
While it simmers, toast the walnuts in a dry 20 cm / 8-inch skillet over medium-low heat for 3 to 4 minutes, shaking the pan, until fragrant and a shade darker. Tip them onto a plate.
Take the pot off the heat and stir in the maple syrup and vanilla. Cover and rest for 5 minutes. The porridge thickens as it sits.
Slice the bananas. Divide the porridge among four bowls and top with the banana, blueberries and toasted walnuts. Serve warm.