Meera Sodha's Tomato Curry

Name: Meera Sodha's Tomato Curry
Cuisine: Asian, Indian
Ingredients: 1¼ tsp fennel seeds; 1¼ tsp black mustard seeds; 1¼ tsp cumin seeds; 1¼ tsp coriander seeds; rapeseed oil; 2 onions, halved and finely sliced; 1¼ tsp salt; 8 fresh curry leaves, plus extra to garnish if you like; 1.2 kg tomatoes (ideally 1kg vine and 200g yellow baby plum); 1½ green finger chillies, very finely chopped; 4 cloves of garlic, crushed; 2½ tsp tamarind paste; 1 x 400ml tin of coconut milk