Bring a large pot of salted water to a boil, adding the quartered lemon for flavor. Cook the shrimp for 2-3 minutes until pink and opaque, then transfer to ice water.

Finely chop celery and red onion while shrimp cools, then pat the shrimp dry.

Combine lemon juice, minced garlic, mayonnaise, Dijon mustard, and dill in a bowl. Whisk until smooth and adjust seasoning.

In a mixing bowl, fold together shrimp, celery, and red onion. Add dressing and toss until coated.

Cover and refrigerate for at least an hour. Garnish with extra dill before serving.