Heat a pan over medium heat and pour in amaranth; cook and stir until toasted and fragrant, 3 to 5 minutes. Remove from heat and cool.
Dissolve yeast in 60ml lukewarm water in a small bowl. Let stand until bubbly, about 5 minutes.
Mix remaining water with toasted amaranth, yeast mixture, plain flour, bread flour and salt in a large plastic container with a lid with a wooden spoon until well combined. Cover and let rise in a warm, draft-free place until doubled in volume, about 2 hours.
Dust a cutting board with 1 tablespoon plain flour. Turn the dough out onto the board and briefly knead with floured hands. Shape into a ball.
Line a large bowl with a piece of parchment paper. Place dough in the bowl and cover with a clean tea towel; let rise until nearly doubled in volume, about 1 1/2 hours.
Preheat oven to 230 C / Gas 8 after dough has been rising for 45 minutes. Place a lidded ceramic or cast iron casserole pot in the oven after 25 minutes.
Remove hot casserole pot from the oven using oven mitts. Place dough carefully inside, lifting it up with the parchment paper. Cut any parchment that is peeking out of the pot with kitchen scissors. Cover with the hot lid.
Bake bread covered in the preheated oven for 30 minutes. Remove the lid and continue baking until top is golden brown, about 10 minutes. Lift bread out of the pot with the parchment paper using oven mitts and tap the bottom; if it sounds hollow, bread is done. Let cool for at least 1 hour on a wire rack.