Heat the olive oil in a medium saucepan over medium-high heat. When hot, add the onions, carrots and celery; cook, stirring, for 3 minutes. Add the lentils, vegetable stock, bay leaf, salt and pepper. Bring to a simmer and cook uncovered until lentils are just tender, 40 to 45 minutes, stirring occasionally. Take care not to overcook or lentils will become mushy. When the lentils are just tender, strain immediately in a colander and discard the cooking liquid.
Place the warm lentils in a bowl and add the red and yellow bell peppers, parsley, balsamic vinegar and extra-virgin olive oil. Toss well to combine.
Adjust seasoning if necessary and allow to cool to room temperature before serving.