Heat 1 1/2 inches peanut oil to 365 degrees F in a pot over medium-high heat. Whisk 1/2 cup each flour and cornstarch, 3/4 cup seltzer and 1/4 teaspoon kosher salt; toss with 1 pound green beans. Remove the beans from the batter and fry in batches, stirring, until golden, 2 minutes. Drain on paper towels. Serve with 1/4 cup soy sauce whisked with 2 tablespoons water, the juice of 1 lime and 1 minced scallion.