Gently saute the onion and garlic in the oil until caramelized. This can take up to 20 minutes, so don't rush this step and don't burn the onion.
Then add all the other ingredients, except the salt and pepper, stir to mix.
Bring to boil, then gently simmer until the soup develops a glossy sheen. This part is hard to describe but you know when it has reached this stage because it develops a 'cooked' look and the watery consistency vanishes. I think the length of time this stage takes depends on the lentils and perhaps how old they are. I've found generally about 1 hour to 11/2 hours is about right, but if you cook it longer, it won't harm the finished product.
Add salt and pepper to taste towards the end of the cooking period. I've heard that adding the salt at the beginning can make the lentils tough and difficult to cook.
Serve with a dash of Greek Yoghurt and Pita bread or toast, if desired.