Heat the oil in a large saucepan over medium high, then gently sauté the onion and garlic until soft, for about 5 minutes.
Add the spices, stir until fragrant then add the tomato purée. Stir for a few second then add the chopped vegetables and coat them well in the spices. Add the lentils and the stock.
Bring to the boil, then turn the heat and simmer for about 20 to 30 minutes, until the lentils are cooked through but not falling apart.
Turn off the heat. Season the stew with salt & pepper, then scatter the chopped herbs on top.
Serve warm.