One day before you plan to boil the crabs: If you have a heatproof, submergible vessel that will hold 1 or 2 gallons of water that may be frozen, fill these with water and freeze until frozen solid.
In a very large (40 to 60-quart) pot combine the crab boil, lemons, onions, garlic, and 6 gallons of water and heat over high heat, stirring, until the powdered seasoning has dissolved. Add the crabs, potatoes, corn, artichokes, smoked sausage and hotdogs. (Everything should be submerged in liquid ñ if not, add a bit more water to cover.) Cover the pot and bring to a boil. Cook at a rolling boil for 6 minutes. Turn off the heat and add the mushrooms. Cover the pot and let sit for 15 to 20 minutes.
Add the frozen vessels to the hot crab boil and stir occasionally very gently to facilitate even cooling. This will prevent the crabs from overcooking and will also force them to absorb the seasoning from the crab boil. Let the crabs sit in the water for at least 1 hour before serving. You can let the crabs sit until completely cool, if desired, or you can serve the crabs warm.
Using tongs or strainers, carefully remove the crabs from the pot along with the onions, garlic, potatoes, corn, artichokes, sausage, hotdogs, and mushrooms and spread out on large platters or on newspaper lined tables for folks to enjoy.
Yield: 4 to 6 servings
Note: We used a 60-quart aluminum pot with a strainer insert and did this over a propane burner outdoors. You could probably get away with a pot slightly smaller, such as a 45- or 50-quart pot, as we had about 6 inches of headspace left in ours. This is definitely do-able indoors, but you may need to do it in 2 large pots on the stove and it will probably take quite a while for the pots to boil.