In a large heavy-bottomed saucepan, pour enough oil to fill the pan about a third of the way. Heat over medium heat until a deep-frying thermometer inserted in the oil reaches 375 degrees F.
Mix the flour, cornstarch, salt and pepper in a medium bowl. Whisk in the water to make a thin batter. Dip the green beans, zucchini and lemons in the batter and carefully place them in the saucepan. Fry the vegetables until lightly browned, 1 to 3 minutes. Drain the vegetables on paper towels and immediately sprinkle with salt. Serve the vegetables warm.