Prepare the prawns: remove shells leaving tails, devein, and score the belly side in 2-3 places about 1cm apart to prevent curling during frying.

Prepare the vegetables: slice the sweet potato, remove mushroom stems, and trim the green beans.

Make the tempura batter: combine the ice-cold water and mayonnaise in a bowl, then add the flour and mix gently. Do not overmix — a lumpy batter makes crispier tempura.

Heat oil to 180°C (356°F). Dip each ingredient in batter and fry until golden and crispy. Remove to a wire rack lined with paper towels to drain.

Make the tendon sauce: combine the soy sauce, mirin, sugar, shirodashi, and water in a small saucepan. Bring to a boil, then simmer until slightly reduced and thickened.

Divide the rice into large bowls. Arrange the tempura on top and drizzle generously with the tendon sauce. Serve immediately while the tempura is crispy.