Sauté onion in oil or butter over medium heat. Then add garlic. Slowly cook stirring every now and again for 10 minutes until tinged with gold and the onion is sweet.
Turn up heat and add the spice or paste, cooking off the spices, stir for 1 minute.
Add remaining ingredients and any other vegetables you with to add such as carrots and stir. Bring to a simmer, then place lid on and adjust the heat to low/medium so it is simmering gently.
Simmer for around 30 minutes then remove lid and simmer for a further 10 minutes to reduce the sauce. The lentils should be soft and the sauce should be thick and creamy. If its too thick add more water and if its too thin simmer with the lid off and it will thicken quickly.
Serve hot with cooked rice.