Finely slice the florets lengthways. To make the batter, mix the dry ingredients and cold soda water together. (Don’t beat it for too long; it should be lumpy.) Heat the oil in a heavy based pot. Dust the florets in the flour, dip them into the batter, and fry in oil until golden brown and crispy. Drain on kitchen towel and keep warm. Mix the mayonnaise and garlic together, season to taste and serve with the tempura vegetables as a dip.