Melt the butter, stir in the tomato paste, stock powder and seasoned ketchup. Put the sliced onions on top and sauté.
Dice the tomatoes, peel them beforehand if necessary and add. Put the washed, salted and soured fish on top. Let it steep in the closed pot for about 7 minutes.
Season the sauce with salt and pepper and, if necessary, thicken it with 1 tablespoon of cold mixed flour if it seems too runny.