Add the lentils to a pan and cover with enough cold water to cover them by about 2 inches. Bring to the boil, skimming off any scum that rises to the top, and then reduce to a simmer. Stir in the turmeric and a generous knob of butter (if vegan, just omit the butter). Cover and leave to cook gently.
In a separate frying pan, dry-fry the cumin seeds over a medium heat until toasted and fragrant. Remove from the pan and leave them to the side. Melt a second knob of butter in the same frying pan (or use oil) and gently fry the chopped garlic, onion, chilli and tomatoes. Once everything is beginning to turn golden, mix in the toasted cumin seeds, the garam masala and ground coriander. Remove from the heat until the lentils are completely softened.
Give the lentils a good stir, you want them to be thicker than soup but not as thick as mashed potato. Stir in your fried onion mix, season to taste and serve with a twirl of yoghurt and some coriander and a side of rice or naan bread.