For the spice paste: Roughly chop the fresh ginger, chili and spring onion and add to a blender or small food processor, along with the ground turmeric and the sesame oil. Blitz to a chunky paste.
Peel and cube the sweet potato. Heat a glug of sesame oil in a large pan set over a medium heat. Add the paste and cook for 5 mins until fragrant.
Add the cubed sweet potato and sauté for a couple of minutes to sear the edges. Add the coconut milk, water and soy sauce and bring to a boil, then turn down to a simmer. Cook for about 10 mins until the sweet potato has softened. Stir through the spinach and cook until just wilted.
Meanwhile, cook the udon noodles following instructions.
To serve, divide the noodles into bowls and pour over the soup. Finish with your chosen toppings.
