Place a 30-quart pot with a strainer basket on the burner, and fill it about two-thirds full with water. Add 1 cup crawfish boil spice, plus the vinegar, butter, celery and garlic, and squeeze each lemon half over the pot. Light the burner, crank the heat to high, and bring to a boil. When the pot is boiling, add the potatoes and onions. Reduce the flow of propane as needed to keep your pot at a lively simmer, but below a raging boil. When it starts to bubble again, cover and cook for 10 minutes.
Next, add the corn, sausage and mushrooms and return the pot to a lively simmer, then cover and cook for 5 minutes. Remove from the pot and keep warm. (I put mine in large baking pans covered with foil and then stashed in a large cooler.)
Finally, add one-third of the crawfish (10 pounds); cook the rest in two separate batches. When the broth just starts to bubble, turn the heat off and cover the pot. After 10 minutes, don oven mitts and check: If the crawfish still float, cover the pot and let them poach for a few more minutes. Once they sink, they’ve absorbed all the spicy broth they can and are ready to eat. Lift out the strainer, let it drain over the pot, then pour the crawfish out onto a newspaper-covered table. Retain the broth in the pot for the next batch.
To cook the remaining crawfish, bring the broth back to a boil, add more boil spice and the crawfish, and when it begins to bubble again, repeat the cooking process.
Tip: Save the crawfish shells to make stock—étouffée, jambalaya, crawfish bisque and other crawfish dishes reach their zenith when enhanced with this stock. Just cover the shells with water in a large stockpot and add a cut lemon, a dozen or so cloves of garlic, a quartered onion and some celery. Toss in a tablespoon of peppercorns, a few bay leaves and a bunch of thyme. Simmer for 40 minutes, then cool and strain. Freeze until needed.