Heat oil in a large heavy pot to 360° (about 10 to 15 minutes). Meanwhile, in a bowl, whisk flour, salt, and seltzer together until smooth. Let batter stand until oil is heated.
To make the dipping sauce, whisk together soy sauce, vinegar, mirin, sugar, sesame oil, lime juice, chile pepper flakes, and scallions. Set aside.
When oil is heated, dip pieces of apple and sweet potato in tempura batter and submerge in oil, four at a time, making sure pieces do not touch. Turn frequently with a slotted spoon until golden, about 1-½ to 2 minutes. Transfer to paper towels. Serve immediately with dipping sauce.
We love this Asian spin on classic fall ingredients, and it’s so easy to make. The dipping sauce is made with mirin, a sweet, low-alcohol Japanese rice wine.