Steam cauliflower florets until tender; cool and refrigerate until needed. In a deep bowl, whisk flour with beer and a big pinch of salt until smooth, dip cooked cauliflower florets in the batter and carefully place one at a time in a deep-fryer, preheated to 360?F; cook until golden brown. (Alternatively, heat a couple inches of canola oil in a pot until it's hot but not smoking - a scrap of bread should sizzle when dipped in. Cook a few at a time, without crowding the pot.) Remove with a slotted spoon, transfer to a paper towel-lined plate and season with salt.

To make the mayo, stir all the ingredients together in a bowl, adding enough juice to keep it thick, yet tangy, and seasoning with salt and pepper to taste. Serve the fritters immediately, with the mayo for dipping.