1 cup dry crab boil seasoning (such as Zatarain's or Old Bay)
1/4 cup kosher salt
2 yellow onions, quartered
6 garlic cloves
3 lemons, halved crosswise
2 1-pounds heads of cauliflower, cored, trimmed into 2" florets
Ingredient info: Crab boil seasoning is available at most supermarkets in the seafood or spice section, and at specialty foods stores.