Simmer mirin, soy sauce, vinegar, and kombu in a saucepan for three minutes until reduced by half.
Cool the liquid, remove kombu, and stir in ponzu or lemon juice.
Pour dressing into bowls and garnish with spring onion and chilli flakes if desired.
Boil asparagus for 90 seconds until tender, then drain and rinse under cold water.
Pat scallops and asparagus dry carefully with kitchen paper.
Heat 4cm of oil in a large saucepan to 180°C.
Mix cornflour, self-raising flour, and sesame seeds in a bowl.
Whisk egg yolk with 100ml water, then gradually whisk into the flour mixture.
Whisk in the remaining 100ml water until just combined, keeping the batter slightly lumpy.
Coat scallops in the batter and fry in batches for 2 to 2.5 minutes until golden.
Drain fried scallops on kitchen paper and repeat with asparagus for 90 seconds to 2 minutes.
Serve tempura immediately with the prepared dipping sauce and lemon wedges.