How to Make It
In a saucepan, heat 1 inch of oil to 360°. In a bowl, whisk the flour and 1 1/4 cups of the seltzer until smooth and the consistency of sour cream; add more seltzer if the batter is too thick. Season with salt.
Working in 3 or 4 batches, dip the squash and some of the sage in the batter; let excess batter drip off. Carefully add the battered squash and sage, and some uncoated sage leaves, to the hot oil. Fry over moderately high heat, turning, until lightly golden and crisp, 3 to 5 minutes. Using tongs, transfer the fried squash and sage to paper towels to drain. Season with sea salt and serve right away with lemon wedges.