In a large bowl, whisk together 1 cup of the flour, 1/2 teaspoon of the salt, and the wine. Make sure the mixture is smooth and set it aside for 30 minutes.
While the mixture sits, prepare the vegetables. Trim and cut the green onions into 2 1/2-inch lengths. Trim and de-string the snap peas, if you like. Rinse any grit from the morels and pat them dry. Clean and trim the baby turnips, leaving the greens attached, if you like. Snap the asparagus and discard the woody ends. Thinly slice half the lemon, reserve the remaining half of lemon to spritz the fritto misto when you serve it.
Heat the oil in a large pot over medium-high heat. If you have a thermometer, use it to know when the oil hits 350°F - 375°F and keep it there. If you don’t have a thermometer, know that the oil is the correct temperature when a piece of battered vegetable sizzles immediately when added to the oil. If it doesn’t sizzle immediately, the oil is too cold. If it sputters or throws off oil, the oil is too hot.
Preheat an oven to 220°F. Set a cooling rack in a baking pan. As the oil heats, stir the seltzer water into the batter and coat the vegetables. Work with 1 type of vegetable at a time. Put the vegetables in the batter, move them around to coat them, and lift them out, letting excess batter drip off. Put each piece in the hot oil and fry, stirring or turning for even cooking, until golden and tender. Only put as many vegetables as fit in a single layer. Time will vary depending on what type and size, but 2-3 minutes is the general range.
Lift the vegetables out of the oil using a slotted spoon, birds nest, or tongs, and transfer to the cooling rack set over a baking sheet to drain. Keep in the warm oven as you cook the remaining vegetables.
As soon as all the batches are done, transfer all vegetables onto a serving platter and serve hot with a spritz of lemon. Season to taste with additional salt, if you like.