To Make the Tentsuyu Dipping Sauce
Add kombu into a saucepan with ¾ cup water. Bring to a boil, remove from heat and add in the bonito flakes. Allow to steep for 10 mins, and then strain through a sieve.
Return this dashi stock back into the saucepan and add remaining sauce ingredients. Bring to a boil, and then remove from heat and set aside to cool.
To Cook the Tempura Vegetables
Heat oil in a saucepan or pot to 170°C.
Add panko breadcrumbs into a bowl.
In a separate mixing bowl, add flour and egg white. Slowly pour over soda water while whisking slowly until just combined with some lumps in the batter.
Dip eggplant and sweet potato slices into tempura batter and then coat in panko breadcrumbs.
Carefully drop into hot oil and fry until crispy and golden brown. Fry in batches until remaining ingredients are finished. Carefully remove and drain on paper towel.
Serve with Tentsuyu dipping sauce.