Bring the masala sauce, lemongrass and coriander stems to the boil in a pan while stirring, pour in the stock.
Add the sweet potatoes to the sauce, simmer over a low heat for approx. 8 mins. Add the peas and beans, simmer for approx. 2 mins. while stirring, season. Tear off the reserved coriander leaves, sprinkle on top, serve with the naan bread.