Preheat the oven to 180°, 350°F, Gas 4.
Heat the sunflower oil in a large casserole over a low-to-medium heat, add the finely chopped onion and the crushed or finely grated garlic, whichever you're using. Cover and cook for 8-10 minutes, until the onions and garlic are soft.
Increase the heat, add the ground coriander seeds, the ground cumin seeds, the black mustard seeds, the grated ginger, the finely chopped chilli, the pinch of ground cinnamon, the crushed cardamom seeds and the turmeric. Stir.
After one minute, add the tomato puree and the red lentils and stir, then add the chicken stock or vegetable stock, whichever you are using. Bring to the boil, season with salt and freshly ground black pepper, cover and bake in the oven for 8-10 minutes or until the lentils are cooked and have soaked up all the liquid.
Add the chopped coriander and the squeeze of lemon and season to taste.