If you can’t find the Japanese curry sauce mix, substitute for this homemade curry paste:
In a saucepan, melt 60 ml (1/4 cup) of butter. Add 60 ml (1/4 cup) of flour and cook for 1 minute, stirring constantly. Add 30 ml (2 tablespoons) of Worcestershire sauce, 30 ml (2 tablespoons) of garam masala and 15 ml (1 tablespoon) of tomato paste. Remove from the heat and let cool.
You can find the Japanese curry sauce mix, in boxes of 100 g or 200 g, in the Japanese product aisle in Asian grocery stores.