Japanese curry is thick, glossy and gently sweet, closer to a rich stew than anything you would call spicy. Give it a morning in the slow cooker and the beef turns meltingly soft while the potato and carrot soak up all that flavour.
The roux block does the seasoning for you, so the only real job is browning the beef and onions properly before everything goes in. That colour is what stops the curry tasting flat.
Stir the roux through at the end, let it thicken, then serve it over rice with a spoonful of pickles on the side. It is the easiest big-batch dinner in the rotation.