Fish must be absolutely fresh. Clean the fish well making sure the gills are removed from the head. Separate the head from the body and remove all bones from fish. Save head and bones.
Place onion, carrot, celery, one slice lemon and the peppercorns into a fish poacher. Add the fish filets, the head and the bones.
Add cold water to cover and 1/2 tsp. salt. Simmer, covered, for 10 - 20 minutes or until done. The fish is done when the eye pops out a little and the meat flakes. Remove from heat.
Carefully pick out any pieces of meat avoiding bones and vegetables and arrange inside a fish mold or on an oval serving plate.
Lightly season with salt, white pepper and lemon juice. Strain the broth and return to the stove.
Let it boil uncovered until the liquid is reduced to about a cup, then pour it over the fish and refrigerate until the gelatin is firm.
To serve, unmold and cut in half lengthwise with a sharp knife. Then cut each half into four or five pieces diagonally forming a fish bone pattern. Mask the cuts under a swirl of mayonnaise so the effect looks like a whole fish. Cut the lemon slices and arrange around the fish to look like fins.