1 gallon water
2 tablespoons plus 1 teaspoon kosher salt
2 pounds large shrimp, peeled and deveined
1 cup sliced onions
3 bay leaves
⅓ cup capers
¼ cup fresh lemon juice
1 cup cider vinegar
½ cup olive oil
1 teaspoon minced garlic
1 teaspoon celery seeds
1 teaspoon hot red pepper flakes
1/2 cup chopped fresh parsley
Heat the water in a large pot over high heat and bring to a hard boil. Add 2 tablespoons of the salt and stir to dissolve. Add the shrimp and boil just until they turn pink, about 30 seconds or so. Drain.
Meanwhile, in a large bowl combine the onions, bay leaves, capers without juice, lemon juice, vinegar, olive oil, garlic, remaining 1 teaspoon salt, celery seeds, pepper flakes, and parsley. Transfer the shrimp to the marinade.
Cover tightly and refrigerate for 8 to 12 hours.