Place a small skillet over medium heat and add the butter. Bring to just foaming. Add garlic and Cajun seasoning and simmer until aromatic, about 2 minutes. Reduce heat to the lowest setting to keep warm. Set aside.
Fit a large pot fitted with a steamer basket and fill with water just beneath the steamer basket. Bring to a boil.
Place a colander in a large bowl in the sink. Add the live crawfish to the colander and fill with cold water. Gently shake to remove excess grit. Drain, and repeat the process until the water runs mostly clear.
Add the potatoes, onions and sausage to the steamer basket and cover. Steam for 10 minutes. Add corn on top of the potatoes and onions and cover. Steam for 5 additional minutes. Finally, add the crawfish on top of the corn and cover. Steam for an additional 5 minutes.
Uncover and carefully lift the steamer basket from the bottom pot, allowing any excess liquid to drain back into the pot. Pour all contents into a large mixing bowl and add the garlic butter. Toss to coat.
To serve, garnish with parsley, scallions, and lemon wedges. Line the table with newspaper to catch any drippings, shells, and finished corn cobs. Serve in a large bowl with a roll of paper towels and wet naps.