Place amaranth, onion, garlic, sea salt and stock in a saucepan. Bring to a simmer, cover and reduce heat to low. Let cook until all liquid is absorbed, about 40 minutes. Transfer cooked amaranth to a bowl and let cool.
Once amaranth has cooled, add egg, flour and chopped basil. Mix well.
Heat oil over medium-high heat about ¼-inch deep in a wide skillet. Drop 2 Tbsp batter per serving into the hot oil. Let cook until browned on one side, about 2 minutes, then turn to cook on other side. Remove from oil and drain on paper towels.
Serve with tomato sauce or salsa.