Sweet Potato and Onion Tempura With Chive Mayo

Name: Sweet Potato and Onion Tempura With Chive Mayo
Ingredients: 1 cup mayonnaise; 1 Tbsp. Dijon mustard; Zest of 1 small lemon; 2 Tbsp. fresh lemon juice; ⅓ cup chopped chives, plus more for serving; 1½ tsp. Diamond Crystal or ¾ tsp. Morton kosher salt, divided, plus more; Freshly ground black pepper; 2 medium sweet potatoes (about 1½ lb.), scrubbed, sliced into ¼-thick rounds; 2 medium yellow or red onions, sliced into ¼-thick rounds; 1¼ cups (156 g) all-purpose flour; 1 cup (122 g) cornstarch; 1 12-oz. can chilled seltzer; Vegetable oil (for frying; about 8 cups)