Put the lentils in 1.5 litres of boiling water, bring to the boil, cover and cook for about 20 minutes. In the meantime peel and finely dice the onions and carrots. Clean and wash the leek, cut in half lengthwise and dice finely. Wash the thyme, shake dry, put something aside for garnishing. Pluck the remaining leaves from the stems
Heat olive oil in a wide saucepan. Add the onions, leek, carrots and thyme and steam for about 10 minutes, turning. Sprinkle with sugar, caramelise slightly and deglaze with vinegar. Bring to the boil and simmer for 4-5 minutes
Drain the lentils well in a sieve and add to the vegetables. Heat again and season with salt and pepper. Arrange the lentils garnished with thyme