Over the lowest heat setting add the whole wheat flour and oil and combine to a paste.
Heat up your pan over a low to medium heat setting. Start mixing and reducing. Keep on stirring all the time. This can take about 8-10 minutes.
Use lowest heat setting or take from heat and mix in salt and sugar so that no lumps are formed.
Use lowest heat setting or take from heat. Add the ground cumin, coriander and garam masala and combine.
Use lowest heat setting or take from heat. Mix in ketchup and soy sauce.
Keep over a medium heat setting and stir continously so that your roux starts to dry. Try to reduce lumps from forming by breaking them with your spatula. Do this for about 5-10 minutes or until you are satisfied with the result.
Allow to cool and place roux in a jar or freeze in ice cube shapes or use diretly in your Japanese curry.
Makes about 1 jar of 10.6 to 12.3 ounces/ 300-350 gram, which should be enough to make 10 curries (2 servings each) or 20 servings.