Fill a medium saucepan halfway with water and bring to a boil. Add lentils and cook 18 - 23 minutes, until tender. Drain.
Meanwhile, bring another medium saucepan filled two-thirds with water to a boil; add celery and blanch 20 seconds. Immediately drain in a strainer and run cold water over celery until lukewarm.
Process lemon juice, garlic, salt, and mustard in a blender on low speed. Gradually drizzle oil through feed tube, increasing speed to high until blended and smooth.
Toss celery, lentils, and parsley with dressing. Add salt and pepper to taste.
Servings 4
* Percent Daily Values are based on a 2,000 calorie diet. Your daily value may be higher or lower depending on your calorie needs.