In a nonreactive 5 1/2 quart pot, combine the onions, celery, garlic, lemon halves, kosher salt, seafood boil, and cayenne. Add the 8 cups of water. ​Bring the liquid to a boil over high heat. Reduce heat and let simmer, uncovered, for 10 minutes.

​Add the shrimp and return liquid to a boil. Remove from heat and let sit, uncovered for five minutes.

​Strain the shrimp and other contents of the pot in a large mesh strainer. Spread the strained shrimp and other ingredients from the strainer on a rimmed baking sheet and refrigerate uncovered until well chilled, at least two hours, before serving. (This can be done early on the day of serving.)

​Serving Suggestion: If the shrimp size is extra-large to colossal, allow three shrimp per person. If the size is large, allow five per person.