Very finely dice the onion and chilies. Grate the ginger. Wash the sweet potato (peel if desired) and dice it into bite-sized pieces. Set vegetables aside and cut the turkey into bite- sized pieces.
Put the black mustard seed, turmeric and fenugreek powder into a large saucepan and heat gently until the aromas are released. Add the oil and ginger, chilies and onion. Stir gently until the onion has softened. Add the turkey pieces and cook for 5-6 minutes until the turkey is browned.
Pour in the water and mix well. Add the sweet potatoes, curry leaves and tomato and simmer for 20 minutes.
If you would like the sauce to be a little thicker, mix the cornflower with a small amount of water to make a paste. Add the paste to the curry, making sure to stir constantly to stop any lumps from forming.
Keep stirring until all is mixed through.
Add the spinach and cook for 2 minutes until wilted.
Serve with brown rice or a whole grain naan.
Wash the brown rice well and put into a saucepan with 150ml water. Bring to the boil and simmer for 5 minutes and then cover with a lid and turn off the gas. Check the rice in 15 minutes and all the water should be absorbed. Fluff with a fork and serve.