Put banana and lemon juice into a bowl. Mash with a fork until very smooth.
Add amaranth in small amounts, mixing after every addition. Continue adding amaranth until you have a stiff dough that holds together when you squeeze it. Dough will be slightly sticky.
Line a baking tray with parchment paper or aluminum foil. Brush the paper lightly with oil. Take another piece of parchment or waxed paper and brush it with oil, too.
Optional: Allow the dough to rest for about 15 minutes. It will become less sticky.
Divide the dough into about 24 equal-sized pieces. Roll each in your hands to form a ball. Place about half the balls on the prepared paper, leaving plenty of room between.
Put the other piece of oiled paper on top of the balls. Press each ball with a flat-bottomed dish to form a circle, about 1/8" (3mm) thick.
Sprinkle the crackers lightly with salt.
Bake at 180°C for about 12 minutes, or until the edges are golden. Remove pan from oven and carefully flip each cracker, then return to the oven for about 12 minutes more.
Tap each cracker lightly in the center with your finger. If the cracker is firm, remove it to a cooling rack. If the cracker feels soft, let it bake a little longer.
Roll, flatten, and bake the remaining half of the dough the same way.
Store in an air-tight container.
Notes
If you want to toast the amaranth yourself you will need a very heavy pan, such as cast iron or stainless steel. It requires very high heat, which can warp lightweight cookware.Heat a frying pan over high flame. Add just enough amaranth to coat the bottom of the frying pan. Cover immediately—amaranth will pop as it toasts. Cook, shaking frequently, for a couple of minutes until about half the grains are golden. Pour toasted amaranth into a bowl. Repeat until all the amaranth is toasted.