Soak soya granules in milk for an hour. Crush a little in mixer. Chop one onion and cut the remaining two into roundels.
Heat two teaspoons of oil in a pan. Add chopped onions and green chillies and sauté. Add soya granules, two tablespoons of tomato ketchup, one tablespoon of mustard paste, salt, coriander leaves and continue to sauté till dry.
Remove from heat and transfer into a bowl. Set aside to cool slightly. Add boiled and grated potatoes and mix properly. Divide into four equal portions.
Take whole wheat flour in a bowl. Add salt, red chilli powder and a little water and mix to make a smooth batter. Take one portion of the soya-potato mixture, shape into a pattie. Dip in the flour batter and roll in brown breadcrumbs. Prepare three more patties.
Heat a little oil on a griddle plate. Place the patties on it and cook. Cut the burger buns. Separate onion roundels into rings. Heat two teaspoons of oil in a pan and sauté the onion rings till golden brown. Drain and set aside.
Heat a tawa and roast the burger buns. Turn over the soya patties to cook on the other side. Apply the remaining tomato ketchup to the burger bases, place halved lettuce leaves and over it place some caramelized onion rings.
Place the soya pattie on top, cover it with a tomato slice and another halved lettuce leaf. Apply mustard paste to the top half of the burger bun and place over it.
Secure with a toothpick and serve.