Note: A crawfish boil requires a large pot. For ease of removing the vegetables and crawfish you may find it easier to use a pasta pentola or similar insert and boiling the ingredients in batches.
In a large, 20 quart or larger, stock pot, fill with 2-1/2 gallons of water or until 3/4 full. Add onions, lemons, garlic, bay leaves, mustard seeds, coriander seeds, allspice, dill, cloves, red pepper, thyme, salt, paprika, black pepper, white pepper, cayenne pepper, basil, and oregano. Stir to combine. Over medium heat, bring water to a boil then reduce to a simmer. Allow spice mixture to cook for 45 minutes to 1 hour.
While mixture is boiling clean crawfish by washing with clean cold water 4 or 5 times. Note: Many traditional recipes call for purging the shellfish with saltwater. This isn't necessary, and, if using live crawfish, will only kill the shellfish. Rinsing multiple time with clean water is all that is necessary.
Add potatoes to simmering liquid. Let cook for 20 minutes. Add corn, and simmer for 10 minutes. Increase the heat, and bring liquid back a boil. Add crawfish, and stir to combine into the boiling liquid. Boil for 10 to 15 minutes depending on their size. .
Turn off the heat, cover and allow the crawfish to sit in the liquid for 20 to 30 minutes. Remove vegetables and shellfish from liquid and serve. .
Dipping sauce (Optional): Remove 4 cups of seasoned liquid from stock pot and strain to remove any loose whole spices. Pour liquid into a medium sauce pot over medium heat, and bring to a rapid simmer. Allow to simmer 15 to 20 minutes, reducing by half. Add 1/2 cup (1 stick) of butter, and let melt in hot liquid, stirring to combine. Once butter is incorporated, remove from heat and serve with hot crawfish.