Line the bottom of a cast iron skillet with a layer of aluminum foil. Turn the heat to medium and lay the cut sides of the onion face-down on the foil. Cook until the undersides are blackened, about 30 minutes. Cool before peeling off foil.
Fill a large (5-gallon) pot with 12 quarts water and bring to a boil. Add the salt, sugar, onion, garlic, orange and lemon halves, coriander, chiles, peppercorns, bay leaves, mustard seeds and potatoes. Simmer about 25 minutes, or until potatoes are almost tender.
Add the andouille sausage and cook about 10 minutes. Add the corn, crayfish and dill, dunking the crayfish down into the water to submerge them. Cover the pot; when the pot comes back to a boil, cook exactly 5 minutes.
With a large strainer, lift everything from the pot onto a butcher- or freezer-paper covered picnic table. Serve immediately, with lemon wedges and a spicy mayonnaise.
Nutrition information per serving: