Fill a large stock pot halfway with water. Squeeze in lemon juice, then toss in the halves. Add Cajun seasoning, bay leaves, garlic, and a generous pinch of salt. Bring to a rolling boil.
Toss in the potatoes, onions, mushrooms, and sausage. Boil for about 10 minutes, or until the potatoes are nearly fork-tender.
Add corn and crab legs, simmer for another 5-7 minutes.
Add shrimp and crawfish tails last-they only need 3-4 minutes. As soon as the shrimp turn pink and curl, turn off the heat. Stir in butter for a glossy, rich finish.
Drain with a spoon or ladel with holes in it, serve on a large tray, drink something cold, and enjoy the moment.