Heat 1.5″ vegetable oil in a deep pot until it reaches 350ºF.
Meanwhile, combine the flour, salt, and cold tonic water in a bowl and whisk until just combined. It’s okay if it still has a few lumps. Be careful not to over-work the batter!
One by one, dip the squash blossoms in the batter until coated. Shake off the excess and drop into the oil. Fry the blossoms in batches and be careful not to over-crowd the pot. Fry for 2 minutes per side until golden brown all over. Transfer to a paper towel-lined plate and sprinkle with salt immediately after they come out of the hot oil.