Place the cubed sweet potato in a saucepan with water and bring to the boil. Cook for 5 minutes and then remove from the heat and drain.
Cut the tofu into cubes and place them in a large dish with the cubed sweet potato and shallots.
Whisk together the tamari sauce, tomato puree, water and ginger. Pour this over the tofu and vegetables. Season and leave to marinate for at least an hour but preferably longer.
Blend the lemon, ginger, garlic, chilli, coconut milk, peanut butter and tamari in a food processor. Pour the sauce into a pan , bring to the boil and simmer for 5 minutes stirring frequently until it thickens. Set aside.
Put a shallot on a skewer, followed by sweet potato and then a cube of tofu and then repeat again. Continue until you use up all the tofu and vegetables on the skewers.
Place the kebabs in an ovenproof dish and brush with the marinade. Cook for 20 -25 minutes on 180C or until all the vegetables are cooked. Turn the kebabs whilst cooking.
Serve with the satay sauce together with freshly chopped coriander.