check out this translated manga, it explains alot about what goes into japanese curryhttp://www.yanime.com/projects/curry/
It's kind of hard to tell whether or not mustard seeds are used in japanese curry, they're usually visible in indian curries, as japanese curries tend pulverize, grind down all the spices so that the sauce appears uniform.
I found a recipe for curry
Ingredients: (sorry you'll have to do your own conversions)
-25g Curry powder each S&B and C&B brands (I'm sure you can play with the curry spices to come up with your own mixture)
-100cc vegetable oil
-200g finely chopped onions
-vegetable oil for sauteeing
-1 tablespoon grated ginger
-1 tablespoon finely chopped garlic
-1 apple (quartered)
-1 banana (quartered)
-1 tomato (seeded)
-1 tablespoon mango chutney
-1 tablespoon ketchup
-300cc brown veal stock (look up any French cookbook for this)
-1500 cc beef boullion (by this I believe they mean a standard dark beef stock, not the cube of beef bouillon we're accustomed to thinking about here)
-1 tablespoon ground cumin (preferably freshly roasted cumin seeds, and ground)
-rock salt (I think regular salt is fine)
-500g beef (sirloin) thinly sliced
1. Make the roux.
-Saute the flour with the vegetable oil over a medium heat. (I would prefer to use some butter at this point, though this recipe only calls for oil). Saute and mix well with a wooden spoon until the flour and oil are well incorporated.
-Add the curry powder mix and work into the roux until well incorporated.
-In a 120-130 C oven, roast the roux for about 2 hours. (This seems to be an important step to bring out the flavors of the spices.)
-After roasting the roux, cool it and work the mixture with a flat headed wooden spoon (or somesuch utensil) to smooth it out.
2. Prepare the other ingredients
-Saute the onions in oil until browned and well caramelized. Set aside.
-Saute the ginger, garlic until fragrant. Add the onion mixture.
-Add enough beef stock to the ginger/garlic/onion mixture to make a thin paste
-In a blender, add the apple, banana, tomato, mango chutney, ketchup,
and the ginger/garlic/onion paste, and blend until smooth
3. Blend the roux
-Add the pureed mixture to the roux over medium heat. Keep mixing.
-Add the veal stock and stir into the roux mixture.
-As you keep stirring the curry sauce, add hot beef stock gradually until you get to a suitable thick sauce
-Simmer for at least an hour, making sure to keep mixing it so the bottom doesn't burn
-Add the cumin, mix, and simmer for another hour.
4. Allow the sauce to "mature"
-The instructions states to let the curry sauce rest for at least 4 days (yes, 4 days) to mature. I'm not sure if this means in the fridge or at room temperature. In my experience, it is left at a cool room temperature, and at some point each day, brought to a simmer for a little bit and allowed to cool down to room temperature again. Yes, four days of this. Perhaps this step is a bit obsessive, but some experts say it's crucial.
5. Prepare the beef and finish
-Saute the beef, salt and pepper, and some curry powder to taste
-When the beef is cooked to about medium, add it to the curry sauce.
6. Eat the damn thing. Over hot rice.
OK, so there it is. Japanese curry from scratch made by an overly obsessive Yoshoku chef. Remember, also that making a proper fond de veau (veal stock) can take 3 days to make, not to mention a proper beef stock. I'll probably use some shortcuts, like a store-bought demi glace sauce. So maybe you'll gain an appreciation of what went into making that plate of Japanese curry next time you eat at a fancy Yoshoku restaurant in Japan (I haven't found one in the US, so no need to ponder that here).