Make Dressing: In a small bowl, whisk together 4 tbsp olive oil, mustard, lemon juice and a dash of salt. Set aside.
Make Lentils: Boil a large pot of water. Add lentils and a few large pinches of salt. Turn heat down to medium and cook for 20 min. Drain and set aside.
Caramelize Onions: While lentils are cooking, heat 1 tbsp of olive oil in a skillet over medium. Add onions and a pinch of salt. Cook until slightly golden and caramelized, about 10 - 15 min. If they start to dry out, add a splash of water.
In a large bowl, mix the lentils, caramelized onions, tomatoes and parsley leaves. Add half of the dressing to the bowl and thoroughly toss salad. Add more dressing as desired.
Let salad sit and cool for 15 minutes or so before serving.