Place 30-quart pot filled with water over high heat. Add lemons, onions, rock salt, red pepper and mustard. Add crab boil. Mix, breaking bags and boil 10 minutes.
Add crawfish, corn and new potatoes. Return to boil 7 minutes, adding beer the last 2 minutes. Cover pot. Turn off heat and allow to simmer 3 minutes. Remove crawfish and apply table salt before peeling.
Heap crawfish, corn and potatoes on platter.
Cajun Country -- New Iberia, Louisiana