Heat oil in sauce pot (choose a pot that has a lid that fits well). Cook onions till translucent then add garlic and cook for a couple more minutes.
Add bay leaf and peppers then the lentils. Stir to coat with oil and heat briefly.
Pour over enough water or stock just to cover the lentils.
Bring up to a boil then break down to simmer.
Add and adjust seasonings to taste, be sparing with the salt as the flavour will concentrate as the lentils cook.
Cover and let cook till the lentils are soft and most of the liquid has evaporated.
The method here is similar to making risotto, you want there to be just the right amount of liquid left in the lentils when you're done, you want them soft but not mushy (they should hold their shape) so you'll keep the water or stock warm and add just enough at a time to keep the simmering going.
Before your last addition of water or stock, add the tomato paste, this will thicken it up quite a bit, this is saved to the end till the end to prevent sticking and burning and to ensure the lentils cook evenly.
Adjust seasoning
Enjoy!