1. Combine Shemin’s Indian Curry Paste and the garam masala in a small bowl. Whisk the yogurt, salt, and half of the spice mixture into a medium bowl; add the chicken and turn to coat. Cover and chill for 30mins or as long as possible. Cover and chill remaining the spice mixture.
2. Heat the oil in a large heavy pot over a medium heat. Add the onion and cardamom, cook, stirring often, until onions are soft, for about 5 minutes. Add the remaining half of the spice mixture and tomato paste and cook, stirring often, for about 4 minutes. Add a little water if the mixture gets too dry.
3. Add the tomatoes and chillies plus 300ml water. Bring to a boil; reduce heat, and simmer, stirring often until the sauce thickens, for about 8-10 minutes.
4. Add the cream and chopped coriander. Simmer, stirring occasionally, until the sauce thickens, for about 15 minutes.
5. Meanwhile, preheat the grill at the highest setting. Line a rimmed baking tray with foil and set a wire rack inside the baking tray. Arrange chicken on the rack in a single layer. Put the baking tray as close to the grill as possible and grill for 6-8 minutes on each side, until nicely charred at the edges.
6. Cut chicken into bite-size pieces if desired; add to the sauce, and simmer, stirring occasionally, until chicken is cooked through.
Season to taste and stir in the lemon or lime juice to taste.
Serve with hot naan bread or rice and garnish with coriander sprigs, cinnamon bark or curry leaves if desired.
Note: The marinade from the chicken is not normally mixed into the tomato sauce.
Do Ahead: Chicken can be made 2 days ahead. Cover; chill. Reheat before serving.