Combine all the ingredients in a large bowl, mix thoroughly. Season with salt and pepper, then chill for at least an hour.
Turn on the BBQ (or heat a griddle pan).
With wet hands, form equal sized balls then flatten them between your palms. Rest them on a plate.
Using a vegetable peeler cut the cucumber into thin ribbons and thinly slice the onion into rings. In a bowl, mix the onion and cucumber, add the lemon juice, then sprinkle with salt and sugar. Rest for about 10 minutes.
Make some raita by mixing Greek yoghurt with chopped mint leaves and pressed garlic.
Brush the burgers with some oil then cook them on each side for about 4-5 minutes, depending on how well done you like them.
Warm up the naan on the BBQ.
Put some of cucumber salad on each naan, add the burgers, then top with some raita and a few mint leaves.
Serve with mango chutney and the leftover raita.