Tandoori Chicken Burger

INGREDIENTS
2 white onions
1 red chilli
1 small knob of ginger
4 boneless chicken thighs
1 tbsp of garam marsala
1 tbsp of curry powder
1 cooked beetroot, no vinegar and peeled
1 tbsp of tomato purée
a handful of plain flour, plus extra for rolling the burger
a glug of olive oil
4 dollops of natural yoghurt
a handful of coriander, chopped
2 naan bread, warmed
½ a cucumber