It is not necessary to soak the lentils in advance.
Rinse and drain the lentils, then set aside.
Prepare the vegetables: finely slice the leek; mince or press the garlic; coarsely chop the tomatoes; chop the celery and carrot into very small dices (about 3 mm); cut the broccoli into small pieces (about 2 cm).
In a pot, sweat the leek and garlic in the olive oil over medium heat about 3-4 min. Add the remaining vegetables, lentils, and broth, then bring to a boil, reduce the heat and simmer uncovered for 25 min or until the lentils are tender.