Boil the whole green gram in five cups of water and salt in a deep non-stick pan till completely cooked and soft. You can also pressure cook till the pressure is released four times (four whistles). Drain and use the stock in some other recipe.
Place the onion in a bowl. Add boiled moong, salt and red chilli powder and mix. Add tomato ketchup and mix well. Add mint leaves and mix. Place four brown bread slices on the table top. Spread the moong mixture evenly on them.
Place a cheese slice on each and cover with another brown bread slice. Heat the grill, drizzle a little oil and place the sandwiches on it. Grill till both the sides are golden.
Cut into triangles and serve immediately.