In a large bowl, combine fresh or thawed shrimp with baking soda, kosher salt, and Old Bay seasoning. Toss gently and let sit for 15-20 minutes.

While the shrimp brine, prepare an ice bath in another large bowl with cold water and ice.

Bring a medium pot of water to a gentle boil and squeeze in lemon juice. Add shrimp and poach for about 90 seconds until pink and opaque.

Transfer poached shrimp to the ice bath to cool for about 5 minutes, then drain and pat dry.

In a mixing bowl, whisk together mayonnaise, Dijon mustard, lemon juice, dill, Old Bay seasoning, celery seed, salt, and pepper until smooth. Fold in chopped celery and red onion.

Gently fold cooled shrimp into the dressing mixture until evenly coated.

Cover tightly and refrigerate for at least 2 hours, or ideally overnight, before serving.