How to Make It
In a blender, combine the mayonnaise with the parsley, vinegar, preserved lemon and a pinch each of salt and pepper. Process until smooth, about 1 minute. Scrape the dipping sauce into a bowl and refrigerate until ready to serve.
In a large saucepan, heat 1 1/2 inches of canola oil to 350° over moderately high heat. In a medium bowl, whisk the flour with the cornstarch, baking powder and 2 teaspoons of salt. Whisk in the sparkling water until a smooth batter forms.
Working in batches, dip the broccoli in the batter, shaking off the excess, and fry until lightly browned and crisp-tender, about 4 minutes. Drain the fried broccoli on a wire rack or paper towels and season generously with salt. Serve immediately, with the dipping sauce.
Preserved lemons are a Moroccan ingredient made from lemons that have been cured in lemon juice and salt. Look for them at specialty food shops.