Prepare amaranth in water according to package instructions. Put the lentils and 600 ml water in a pot. Bring to the boil covered and cook over a low heat for about 15 minutes.
Mix the lentils and amaranth and leave to cool. Peel and wash the potatoes and simmer in boiling salted water for about 20 minutes.
Drain the potatoes, let them evaporate and press them through a potato ricer to the amaranth-lentil mixture. Peel and finely dice the onions. Wash parsley, shake dry and chop finely. Add onions, parsley, breadcrumbs, salt and pepper to the amaranth mixture and mix.
Shape the mixture into approx. 8 balls (approx. 125 g each), press flat and chill.
Peel and finely chop the garlic. Halve avocado and remove core. Remove the flesh from the skin and dice roughly. Coarsely puree avocado, garlic and lemon juice with a hand blender, stir in yoghurt, season to taste with salt and pepper.
Wash and slice the tomatoes. Peel onions and cut into fine rings. Heat 1 tablespoon of oil in a frying pan, sauté the onions in it, sprinkle with sugar, allow to caramelize and deglaze with vinegar.
Heat the remaining oil in a frying pan. Fry the amaranth patties in it in portions, turning on each side for about 3 minutes. Roast the toasties, spread the avocado cream on the lower halves, cover with tomato slices, place the amaranth patties on top, spread the onions on top and cover with a lid.