1. Pulse garlic in food processor until finely chopped. Add mayonnaise, zest and juice and process until smooth. Transfer to a bowl, cover and refrigerate until ready to use. (Makes about 1 cup.)
2. Bring a pot of salted water to a boil. Have a large bowl of ice water ready. Add cauliflower to pot; cook until tender, about 3 minutes. Use a slotted spoon to transfer to ice water. Drain.
3. Preheat oven to 200°F; place a baking sheet in oven. Fill a pot with 2 inches of oil; warm over medium-high heat until it reaches 350°F on a deep-fry thermometer, 10 to 12 minutes. As the oil is heating, in a bowl, whisk flour, baking soda, baking powder, Parmesan, 3/4 tsp. salt and 1/4 tsp. pepper. In a small bowl, whisk egg, buttermilk and parsley. Stir into flour mixture. Fold in cauliflower.
4. Using a spoon, drop coated cauliflower into oil and fry in batches, flipping halfway through, until golden brown, 2 to 3 minutes per batch. Transfer to a paper towel-lined plate, then to sheet in oven. Return oil to 350°F; repeat with remaining cauliflower. Serve with sauce and lemon wedges.