To make the curried roux, melt the butter in a pot over medium heat, add in the flour and stir to combine. Keep stirring until the mixture turns slightly golden, about 1-2 minutes. Mix in the Japanese Curry Blend and cook for a further 30 seconds. The roux should be crumbly. Transfer to a bowl and set aside.
To make the curry sauce, into the same pot, add the butter, followed by the onion and garlic, and sauté for a few minutes. Then add the tomato paste and stir through for 30 seconds. Next, mix in the roux and the beef stock, stirring continuously and slowly as you pour to break up any clumps. Add the grated apple and bring to a simmer.
Cook gently for 5 minutes, making sure to remove any impurities that may rise to the top, then mix in the Worcestershire sauce, kecap manis and dark soy sauce. Turn off the heat and set aside.
To a hot frypan over medium heat, add 1 tbsp of the oil and sauté the mushrooms until browned, about 4-5 minutes. Season to taste and then stir through the carrot and potato, sautéing for another 4-5 minutes.
Transfer the vegetables to a slow cooker.
In a bowl, coat the beef in some flour and season with salt and pepper. Add the remaining oil to the same frypan, then add the beef and sear over high heat until browned, about 1-2 minutes per side. Place the meat over the cut vegetables in the slow cooker.
Cover the meat and vegetables with the curry sauce, then set the slow cooker on low and cook until the beef is tender, about 8 hours. Stir occasionally in the last hour to prevent the sauce from sticking to the pot. Adjust the seasoning to your liking by adding some extra soy sauce. Serve with udon noodles and top with a hard-boiled egg, pickled ginger and edamame beans.
