Heat oven to 150°C. Peel your shallots, and then slice them horizontally to make thin disks. Next, separate the disks into individual rings.
Mix together the dipping sauce ingredients in a small bowl.
Cover a baking tray with sheets of kitchen roll. Start to heat a deep-fat frying pan or large wok a third full of oil and have a frying basket, or slotted spoon to hand.
Mix the plain flour, corn flour and ½ tsp fine sea salt in a large mixing bowl. Whisk in the ice-cold sparkling mineral water along with a few ice cubes using a whisk, but don’t over beat. Use immediately.
When the oil is hot dip some of the shallot rings briefly into the batter, shake off any excess, then lower straight into the hot oil. Don’t overcrowd the pan. Fry for about 2 mins until light golden and crisp, then drain on kitchen paper. Repeat with the remaining shallot rings.
Keep the tempura warm in the oven, leaving the door slightly ajar so that they stay crisp. They are best served immediately on a warm plate with the sauce alongside for dipping.