Fill a large pot with water and add a generous pinch of salt. Bring to a rolling boil over medium-high heat. Add the quartered lemon, then drop in the shrimp and cook for 2-3 minutes until bright pink and opaque. Remove from heat immediately.

Transfer the cooked shrimp to a bowl filled with ice water for 3-5 minutes to halt cooking. Drain once completely cool.

In a medium bowl, mix mayonnaise, minced garlic, Dijon mustard, and chopped dill. Squeeze in juice from boiled lemon quarters and season with salt and pepper, whisking until smooth.

In a large salad bowl, combine cooled shrimp, diced celery, and minced red onion. Gently fold in the dressing to coat without breaking the shrimp.

Cover and refrigerate for at least 1 hour before serving to meld flavors. Serve chilled.