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This sauce recipe is from Recipe Tin Japan
40ml Bulldog tonkatsu so-su Sauce
50ml Bulldog usutar so-su Sauce
1 tsp soy sauce
½ tbsp tomato sauce / tomato ketchup
1 tsp sugar
½ tsp dashi seasoning powder diluted in ½ tsp hot water
The powdered seasoning can be okay, but it's nice to know what's in your sauce, and this leaves the noodles less dry.

I'll try to whip that up some time. Thanks!