How do I make Japanese curry? I open a box of S&B Golden Curry Sauce Mix. This is one of the few times where I won't advocate making it from scratch. And you know what? All the Japanese people I know open the same box :)
You start by sauteing chopped onion and cubed beef (or chicken or pork) in oil. Add carrots, celery and/or bell pepper chunks if you wish. Add water, bring to a boil, cover, reduce heat, and simmer for 10 minutes. Remove from the heat, and stir in the curry paste until it dissolves. The box directions say to serve immediately, but I always cook it for a minute or two, just to get rid of the raw flour taste and to thicken it a bit.
Serve over rice or noodles. Easy, spicy, delicious. The fat content of the paste is rather high (64%!), but remember you're diluting that with water, protein, and veggies, so it ends up not being that bad.