How to Make It
Bring 7 quarts water and 2 tablespoons of the salt to a rolling boil in a large stockpot over high. Carefully add shrimp, and cook until the shrimp turn pink and are just cooked through, 1 to 2 minutes. Drain shrimp in a colander, and rinse with cold water. When shrimp are cool enough to handle, peel and devein. (Save shells for stock, if desired.)
Layer shrimp and onion slices in alternating layers in a large, sealable storage container.
Whisk together olive oil, lemon juice, capers, horseradish, Worcestershire sauce, garlic powder, paprika, celery salt, cayenne, and remaining 1 teaspoon salt in a large bowl. Pour marinade over shrimp; cover and chill 2 hours. Toss mixture (or invert container), and chill 2 to 3 days, tossing occasionally. Taste shrimp before serving, and add salt, heat, or lemon juice as desired. Serve as an appetizer, with toothpicks, hot sauce, and lemon wedges. Shrimp will keep for an additional 3 days in the refrigerator.
Note: This recipe can be enhanced with any fresh herbs you have on hand. Feel free to add a few fresh bay leaves to the marinade, or toss marinated shrimp with chopped fresh flat-leaf parsley just before serving.
The total time includes 2 days marinating.