Rinse the lentils in a sieve until the water runs clear.
Tip the lentils into a large saucepan and add the water, garam masala, turmeric and salt. Bring everything to the boil then reduce to a simmer and cook for 60-75 minutes, loosely covered. Skim off any foam that rises in the first few minutes of cooking.
After the yellow split peas have been cooking for 30 minutes, start to make the onion mix.
Heat the oil and butter in a frying pan then add the onion and fenugreek leaves. Fry gently for at least 30 minutes until softened and starting to turn pale golden. Stir often.
Add the garlic to the onions and cook for another 8-10 minutes until everything is golden brown.
Add the ginger and chilli and cook for another five minutes. Finally, add the tomato and fry for five minutes, ensure everything is mixed together well.
When the lentils are cooked and thickened, stir through the onion and tomato mixture with plenty of black pepper. Taste and add a little more salt if needed.
*replace the butter with rapeseed oil to make the recipe vegan