A part of the fish carcass that is often overlooked during the filleting of a fish is the spine itself. A thrifty cook will notice that there can be a good bit of meat left on the spine when fish fillets are removed. Using a spoon, you can scrap off the meat, chop it coarsely, and use it to make tartar.
Chop salmon and scallops coarsely into about 1/8 inch dice. Set aside
In a medium mixing bowl, combine Dijon mustard, lemon juice, Worcestershire sauce and egg yolk and whisk to mix well. Continue whisking while adding oil in a slow steady stream until dressing emulsifies. Add capers and onion and mix well. Season to taste with salt and pepper.
When ready to serve, add salmon, scallops and dill and mix well.
Peel and seed avocados and cut into thin slices lengthwise. Fan avocado slices out on a serving platter. Pack salmon mixture into a bowl that is just big enough to serve as a mould. Invert onto center of platter and remove bowl. Garnish with salmon caviar, if using, and sprigs of dill. Serve with toast points or melba toasts on the side.
Serves 4 to 6 as appetizer.
Note: Alternatively, you can fan half an avocado onto each of 4 plates, and using two spoons, shape a portion of salmon tartare into oval-shaped quenelles and arrange on plate and serve.