Steam the scrubbed mussels in a large pot with ½ cup of water, covered, over medium-high heat for 4–5 minutes until they open. Discard any that remain closed. Remove the meat from the shells and pat completely dry with paper towels.

In a bowl, whisk together flour, cornstarch, baking powder, garlic powder, smoked paprika, salt, and pepper. Add the beaten egg and slowly mix in the cold sparkling water until you have a smooth batter that coats the back of a spoon.

Pour vegetable oil into a heavy-bottomed pan to about 2 inches deep. Heat over medium-high until it reaches 375°F, or until a small drop of batter sizzles and floats immediately.

Working in batches of 8–10, dip each mussel into the batter, let excess drip off, and gently lower into the hot oil. Fry for 2–3 minutes, turning once, until golden brown all over.

Remove with a slotted spoon and drain on a paper towel-lined plate. Let the oil return to temperature before frying the next batch.

Season immediately with a pinch of salt and a squeeze of fresh lemon while still hot. Garnish with chopped parsley and serve right away with your dipping sauce of choice.