1. To make the batter, whisk the flour, salt, egg yolk, and 1/2 cup water in a medium bowl. Add the sparkling water and then the olive oil. Set aside for 20 minutes.
2. Meanwhile, stir together the harissa and mayonnaise in a bowl and set aside.
3. For the frito misto, bring 3 inches canola oil to 375 degrees F in a large, heavy-bottomed deep pot. Line 2 plates or a rack with paper towels.
4. Whisk together the egg whites and salt until they hold stiff peaks. Fold into the batter. Pat dry the vegetables and prawns and dip, a few at a time, into the batter and then into the oil. Fry in small batches, starting with the sage and ending with the prawns, until golden and the vegetables are crisp-tender, about 3 minutes, depending on the size, turning to brown on both sides. Drain on paper towels and season with salt while warm. Serve with the harissa mayonnaise.
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