Bring a pot of salted water to a boil. Add the quartered lemon to extract its citrus flavor into the water.

Add the shrimp to the boiling pot and cook for 2-3 minutes until they are pink and opaque.

Transfer the cooked shrimp to a bowl of ice water to halt cooking and preserve their tender texture. Drain the shrimp and pat them dry.

In a bowl, combine the shrimp, celery, red onion, lemon juice, salt, pepper, mayonnaise, mustard, and dill. Mix well.

Garnish with additional fresh dill and serve immediately, or cover and refrigerate for later.