Unless you live in the bayou, or near a body of high-calcium, fresh water oxygenated by an underground spring, you probably buy your crawfish frozen. Crustaceans' internal organs decompose rapidly after death, so you can only buy precooked, whole frozen crawfish. This doesn't pose a problem, though, because the individually quick-frozen, or IQF, techniques used by suppliers leave crawfish meat soft and nearly pristine, so you don't have to worry about mushiness when reheating. You don't have to thaw crawfish before cooking them, either, so you can add them to crawfish boils and steamers and have them ready in minutes.
City Crawfish Boil
Fill a large stockpot with a few gallons of water.
Add aromatics, herbs and spices to the water.
Crawfish boil cooking liquid usually comprises ground spices such as pepper, cloves, coriander, bay leaves and cayenne pepper; herbs such as parsley and thyme; and aromatics, such as sliced lemons, onions and garlic. You can also buy crawfish/crab boil spice mixes in supermarkets, or put together your own with the spices you like.
Stir the cooking liquid, and season it to taste with kosher salt. Cover the pot, and bring the cooking liquid to a boil on the stove.
Add new potatoes and half ears of corn to the pot, if desired, and cover. Cut potatoes larger than 2 or 3 inches in half.
Cook until the potatoes pierce easily with a fork, about 10 to 15 minutes. Add the frozen crawfish to the pot and cover.
Cook the crawfish until heated through, about 8 to 10 minutes after the water returns to a boil. Drain the crawfish boil in a colander, and serve immediately.
Steaming Frozen Crawfish
Fill a pot about 1/2 to 3/4 full of water, and add whole spices to taste.
You only need to add ingredients that aromatize when heated, such as star anise, crushed garlic, peppercorns and lemon grass. Dried spices and herbs don't release enough aroma into water to have an impact on the crawfish during steaming.
Place a bamboo steamer on top of the pot, or insert a steamer basket in it. Cover the pot and bring the water to a boil.
Separate the crawfish if frozen together, and place them in the steamer. You can stack the crawfish on top of each other, just don't pile them so high the lid won't seal the pot.
Place a kitchen towel over the top of the pot, and place the lid on top of the towel. Press the lid down firmly to keep as much steam in as possible.
Steam the crawfish until heated through, about 15 to 20 minutes. Take a crawfish out of the steamer and cut the tail in half crosswise. If you see steam rising from the meat it's ready to eat.
You can boil frozen crawfish without the carawfish boil ingredients. Just add the frozen crawfish to a pot of boiling water, and cook until heated through, about 8 to 10 minutes after the water returns to a boil.
Don't freeze or reheat thawed crawfish.