Microwave the fish for 3 minutes with a little of the skim milk. Microwave the shallots for 2 minute. Put a saucepan on high heat and add: the stock; the milk; the fish; the onions; ⅔ of the crab; and the tomato purée. Stir the pan to mix. Blend the cornstarch with a little water (adding slowly) and then add the blended cornstarch to the pan as it just starts to bubble. Bring to the boil and then cover, turn down the heat and simmer for 3 minutes. Liquidize the mix in a blender (in bat...