Cook the lentils (boil large pot of salted water, add lentils, turn heat to medium, and cook until the lentils are cooked through, about 20 minutes. Drain the lentils, place them in a large mixing bowl and set aside).

In a smaller bowl, whisk together the mustards, lemon, vinegar, olive oil, and a large pinch of salt. Add the vinaigrette to the lentils along with the onion, tomatoes, and parsley. Season the salad to taste with salt and more lemon or olive oil if you like. This salad is best if you let it sit dr a half hour an served at room temperature.