If not already prepared, poach the fish in a salted, vinegary, vegetable poaching liquid (as for the classic poached fish), then let it cool completely in the liquid, peel off the skin, and slice it.
Pour a layer of the still-liquid but not hot aspic, about 2 fingers thick, into a mold (e.g. a bundt or rectangular mold).
Set the mold in ice water or in the fridge, until the aspic sets.
Arrange radish slices, egg slices, and small salad leaves on the set aspic, for decoration.
Pour a little more aspic over it, and chill again until set.
Once this layer has also set, add the sliced, fully cooled, poached fish.
Fill the whole thing up with the remaining aspic, and chill (in the fridge, not the freezer!) for at least 3-4 hours, until fully set.
To serve, briefly dip the mold in hot water, then turn it out onto a platter.
Garnish with green salad, hard-boiled egg, and radish, and serve.
Based on: Malatinszky Fanny szakácskönyve (a classic Hungarian cookbook), modernised. Calories and protein are estimated.