Fritto Misto

Name: Fritto Misto
Ingredients: Vegetable oil (for frying; about 8 cups); 1 cup all-purpose flour; 1 cup cornstarch; 1 teaspoon baking powder; ½ teaspoon kosher salt, plus more; 2 cups chilled club soda; ¼ small squash (such as kabocha), scrubbed, very thinly sliced; ½ small fennel bulb, very thinly sliced lengthwise; 1 leek, white and pale-green parts only, halved lengthwise, layers separated; 2 ounces maitake or shiitake mushrooms, trimmed, torn into bite-size pieces; 1 lemon, very thinly sliced into rounds, seeds removed, plus wedges for serving; 4 ounces large shrimp, peeled, deveined, halved lengthwise; 4 ounces squid or baby octopus tentacles; ½ cup fresh flat-leaf parsley leaves with tender stems; ¼ cup fresh sage leaves; A deep-fry thermometer