First, make the burgers. Tip all the ingredients into a bowl with a good pinch of salt and squeeze together with your fingers. Shape into three patties and chill in the fridge.
For the raita, grate the cucumber and squeeze out as much liquid as you can. Put in a bowl with the yogurt, mint and some salt. Mix and chill.
For the bhajis, tip the flour, spices and bicarb into a bowl with a pinch of salt. Slowly add 200ml cold water to make a thick batter (you might not need all of it), then mix in the onion.
Heat the oil in a large frying pan. Add the batter in batches so you have three burger-sized bhajis. Sizzle for 3 mins until crispy on one side, then flip and cook on the other side until cooked through. Keep warm.
Barbecue, griddle or pan-fry the burgers to your liking. Put lettuce and a burger on each bun and top with some raita, a bhaji and red onion. Serve the mango chutney on the side.