Katsu Curry Sauce

Melt the butter in a heavy-bottomed saucepan or Dutch oven over medium heat. Add the sliced onions with a pinch of salt and cook, stirring every 2 to 3 minutes, for 15 to 20 minutes total. The onions will first turn translucent, then golden, then begin to develop amber patches and a sweet, almost nutty aroma. Do not rush this step with high heat — the slow caramelization builds the sweet foundation that distinguishes Japanese curry from sharper Indian-style curries. After 15 minutes, add the minced garlic and cook for 2 more minutes.

Add the grated apple to the caramelized onions and stir for 1 minute to cook out any raw edge. Pour in the chicken stock, tonkatsu sauce, and soy sauce. Bring to a gentle simmer over medium heat, scraping up any caramelized bits from the pan bottom — these are pure flavour. Simmer uncovered for 10 minutes, stirring occasionally, until the apple has dissolved into the sauce and the liquid has reduced slightly. The base should smell rich, sweet, and savoury with no sharp raw apple note remaining.

Break the curry roux blocks into pieces and add them to the simmering base off the heat. Stir with a wooden spoon until each block has fully dissolved and no lumps remain — this takes 2 to 3 minutes of patient stirring. Return to very low heat and simmer gently for 10 minutes, stirring frequently to prevent scorching on the bottom. The sauce will thicken dramatically as the starch in the roux cooks out. It should coat a spoon heavily and pour in a slow, controlled stream. Taste and adjust with a pinch of salt or a splash of soy sauce if needed. The finished sauce should taste deeply savoury, mildly spiced, and sweetly rounded — not salty or one-dimensional.