These days, most Japanese households use blocks of instant curry roux that look a bit like a giant chocolate bar. They're produced and sold by brands such as SB Foods, House Foods, and Glico, and they come in sweet, medium, and spicy varieties. The curry is made by sautéing onions and then adding a protein (such as chicken, pork, or beef), carrots and potatoes along with water before cooking it until the meat and potatoes are tender. The roux blocks go in at the very end to season and thicken the curry sauce. Although they taste great, these Japanese curry roux blocks are often loaded with hydrogenated fats, sugar, and MSG, as well as other questionable additives. This is why I've been working on a recipe for making Japanese Curry from scratch for over 10 years. The rich flavor is accomplished by thoroughly caramelizing the onions, and I have a few hacks to help out this process. I also have a great method for chopping onions here.