1 pound assorted vegetables, such as green beans, scallions, acorn squash, cauliflower, avocado, sweet potato, etc.
1 1/2 cups cake flour, divided
2 tablespoons cornstarch
1 teaspoon kosher salt, plus more for seasoning
1 large egg yolk
1 cup ice-cold club soda
2 cups peanut or canola oil
Spicy Mustard Dipping Sauce
1/4 cup mayonnaise
1 tablespoon English mustard (such as Colman’s)
1 teaspoon Sriracha
1 1/2 teaspoons soy sauce
1 tablespoon seasoned rice vinegar
Whisk together dipping sauce ingredients until smooth. Refrigerate until ready to serve.
Clean and prepare the vegetables. Trim stems from green beans. Trim scallions into 3 to 4-inch pieces (slice in half lengthwise if very large). Peel and thinly slice root vegetables like acorn squash, butternut squash, or sweet potato. Cut avocado into long wedges. Trim cauliflower or broccoli into medium size florets.
Add 1/2 cup cake flour to a small bowl or shallow dish.
In a medium bowl, sift together remaining 1 cup cake flour, cornstarch, and salt. Add egg yolk and club soda and stir gently with chopsticks - do not overmix! You want a lumpy batter with pockets of dry flour.
Heat oil in wok over medium heat until it reaches 350˚F. (Safety tip: be sure to turn the handle of the wok toward the rear of the stove so you don’t accidentally bump it while frying).
Working in batches, dredge 4-6 pieces of vegetables in the plain cake flour; shake off the excess, then dip in tempura batter. Transfer to wok and fry at 350˚F until crispy and tender. Cooking times will vary with each vegetable: 1-2 minutes for tender vegetables like green beans, green onion, avocado; 2-3 minutes for heartier vegetables like squash and cauliflower. Transfer to a plate lined with paper towels and season with salt. Continue frying remaining vegetables, adjusting the heat as necessary to maintain a temperature of 350˚F.
Serve immediately with Spicy Mustard Dipping Sauce.