Gut the plaice, rinse it, cut it into thick pieces and place them side by side in a “tinned copper bowl”. Add a good piece of butter, a little grated sage, nutmeg, pepper, rusk, a dash of lemon juice or a slice of lemon and a little water. Steam the fish in it for a good 1/2 hour, “with fire from below and a lid with a lot of fire from above, so that it gets a nice brown color”.
Of course, you can roast a plaice just as well on a broiler or in a frying pan.
For the sorrel sauce, stew the sorrel leaves in a little water until soft. Add a good piece of butter and a pinch of salt. Add some white bread and bring it to the table with the baked plaice.
For the sour egg sauce, mix the egg with the vinegar and mustard. Add the potato flour, stirring until there are no more lumps. Then add the boiling potato water and let the sauce bubble up, boil a little and let it cool down. If the sauce is too thick, you can add a little more potato water. Pour in a drop of salad oil and let a piece of butter melt in the sauce. This sauce, which can be served with fish as well as with leaf salad, is prepared without salt! However, you should not keep it.