Bring a large pot of salted water to boil, add the lentils, turn the heat to medium.

Let it cook just until the lentils are cooked through, about 20 minutes.

Drain the lentils, place them in a large mixing bowl, and set them aside.

In a smaller bowl, whisk together the mustards, lemon juice, 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.