In a large bowl, combine the pork and veal. Mix the wine, brandy and oil with salt and pepper to taste, and pour the mixture over the meats. Scatter the onions and garlic on top. Cover the bowl tightly and refrigerate it for at least 24 hours.
Preheat the oven to 375-F. Discard the onions, but put the garlic through a press and knead it into the meats together with the wine mixture. Break off a small piece of meat and fry it in a lightly oiled skillet over moderate heat for 3 or 4 minutes, or until its juices run clear, without a trace of pink. Taste the piece and, if you like, add more garlic, salt and pepper. (Pork is unsafe to eat uncooked; do not taste the meats raw.)
Slightly overlapping the slices, line the bottom and sides of a 2-quart terrine mold or a 7 1/2 x 3 1/2 x 2 1/2 inch loaf pan with the fatback. Pack the meat mixture into the mold, and arrange the remaining slices of fatback on top of it. Fit foil over the mold, or cover the meat mixture with foil and a lid. To let steam escape, pierce a hole in the foil with a skewer. If you are using a lid, insert the skewer through its hole to puncture the foil.
Set the mold on a rack in a large pan or dish. Place them all in the oven, and pour enough almost-boiling water into the pan to cover 2/3 of the mold. Bake for 2 hours or until the pate shrinks slightly from the sides of the mold and the surrounding fat and juices are a clear yellowish white with no traces of pink. Or insert a meat thermometer; it should register 160-F when the pate is done.
Take the pate from the oven, but leave the foil in place. Set the pate on a rack to cool to room temperature. Then put another pan with a heavy can or weights inside it, or even a brick, on top of the pate to compact the meats. Chill the pate thoroughly (overnight is best) with the weights in place. Before serving, cut the cornichon into a fan shape, slicing lengthwise through the pickle 4 or 5 times to within 1/2 inch of one end. Spread the slices into a fan, and garnish the pate with it.
Servings: Makes about 3 lbs