Begin by washing all the vegetables thoroughly under cold water. Slice the cucumber into thin half-moons, julienne the carrots, and thinly slice the celery.

In a small bowl, combine lemon juice, olive oil, and Dijon mustard. Whisk until emulsified and smooth, about 30 seconds. Season with salt and pepper as needed.

In a large mixing bowl, add the cucumber, carrot, celery, and red onion. Toss the vegetables together to ensure everything is evenly distributed.

Drizzle the prepared dressing over the mixed vegetables and gently toss to ensure each piece is well-coated with the dressing.

Sprinkle chopped parsley over the salad, tossing gently to incorporate. The fresh parsley will add flavor and color.

Allow the salad to sit for 5 to 10 minutes at room temperature before serving to let the flavors meld.