Place the lentils in a large saucepan, pour 3 cm of water over the lentils. Place on medium heat and bring to a boil; cook for another 10 minutes, and then tilt the lentils onto a sieve.
Finely chop the onion, and grate the carrots. Pour olive oil into a saucepan and put on medium heat. Add garlic, onions and carrots and cook for about 5 minutes. Add lentils, 1 liter of water, oregano, rosemary and bay leaf. Bring to a boil. Reduce heat to low, cover and simmer for 10 minutes.
Add tomato paste and season to taste with salt and pepper. Cover and cook for about 30 minutes, stirring occasionally. If the soup gets too thick, add more water.