Slice the zucchini into ¼-inch rounds, cut carrots into thin sticks, and dice sweet potatoes into matchsticks. Pat them dry with a towel to remove excess moisture, which helps prevent splattering.
In a mixing bowl, whisk together the ice-cold sparkling water, rice flour, and cornstarch until just combined. The batter should be lumpy and cold—avoid overmixing to keep it light and airy.
Pour vegetable oil into your deep-frying pan or wok, filling it enough for the vegetables to float freely. Attach a thermometer and heat the oil over medium-high heat until it reaches 180°C (356°F). Test the temperature by dropping a small batter piece—if it sizzles immediately and turns golden in 30 seconds, you're ready.
Dip a vegetable piece into the batter, letting the excess drip off gently. Carefully lower it into the hot oil using a slotted spoon, working in small batches to avoid overcrowding. Fry for 2-3 minutes, turning as needed, until the tempura is a light golden color and crispy.
Monitor the frying process, maintaining the oil temperature around 180°C. The batter should crackle and float, turning a beautiful golden hue. Adjust the heat if the oil begins to smoke or the batter darkens too quickly.
Use a slotted spoon to lift the fried tempura out of the oil and transfer it onto paper towels. Let it rest for 1-2 minutes to drain excess oil and preserve its crispness.
Repeat the frying process with all remaining vegetables, maintaining the oil temperature and working in small batches. Serve the tempura immediately while it’s still hot and crunchy for the best experience.
