Preheat the oven to 200 C, !80 Fan, Gas 6. Remove the tough outer leaves from the lemongrass and trim off the top end. Bash it with the end of a rolling pin then chop as finely as possible.
Put the lemongrass , onion, ginger and garlic in a large oven dish or tray. Add the sweet potatoes then sprinkle over the curry powder, cinnamon and a teaspoon of salt.
Pour in the coconut milk then the stock or water and stir until well mixed. Add the cherry tomatoes on top- keep them on the vine if this is what you have. Place in the oven for 30 minutes.
REmove from the oven and add the black and green beans. Cook for another 15-20 minutes or so until the sweet potato is soft. Check seasoning and add salt and pepper as needed. Serve with rice.