Put split peas in large pan with turmeric and 1.5litres water. Cut ginger in half, finely chop one portion and set aside. Leave other half whole and add to pan.
Bring split peas to steady simmer, half-cover pan with lid, and cook for about 1hr30mins until completely tender and creamy in texture. Add dash more water if nec. Scoop out and discard ginger.
Meanwhile, heat oil in frying pan and add cumin seeds and chilli flakes, cooking for about 30secs until they release warm, spicy aroma. Add onion and fry until golden.
Stir in garlic, chopped ginger and green chilli. Continue cooking for another 2mins, then add garam masala and tomatoes. Fry for another minute, then tip tarka mixture into hot split peas. Stir in chopped coriander, season and serve with rice or flatbread.