To make the dressing, whisk together Dijon mustard, red wine vinegar, and salt and pepper. In a steady stream, whisk in the olive oil. Store in the fridge until ready to use.
Add lentils to a small pot with 1 1/2 cups water. Bring to a boil, cover with a lid and reduce heat. Simmer until they are tender (15-20 minutes). Store in the fridge until ready to use.