Heat the olive oil in a large pot over medium heat. Add the beef cubes and brown them on all sides for about 5 minutes.
Stir in the sliced leeks, diced carrots, and minced garlic. Cook for 3 minutes until the leeks soften slightly.
Add the rinsed green lentils, beef broth, and dried thyme. Bring the mixture to a boil.
Lower the heat, cover the pot, and simmer for 45 minutes until the beef is tender and the lentils are cooked through.
Stir in the Dijon mustard. Season with salt and black pepper to taste.
Ladle the stew into bowls and sprinkle with chopped fresh parsley before serving.