Trim any white membranes from the fish. With tweezers or pliers pull out any bones left in the fillet (this makes slicing much easier). Lay the onions, fennel seeds, sugar and salt in a non-aluminum pan just large enough to hold the fillet flat, such as a bread pan. Lay the fillet on the seasonings, skin side up. Pour the grappa over the fish. Cover first with plastic wrap, then with foil and lay a 3- to 5-pound weight over the length of the fillet (try a sack of flour, bottles, milk cartons, or a large plastic bag of dry beans). Marinate the salmon in the refrigerator for 2 to 4 days, depending on the thickness. When done the fish will look opaque and taste cured throughout.

Remove the salmon and brush, do not wash, the seasonings off. (The gravlox can be rewrapped in plastic and stored for up to 3 days, or served at once.) To serve, cut the meat of the fish crosswise into very thin slices lifting them off the skin which should remain intact. Arrange the slices on a plate and dab each slice with a little of the marinade and some of the olive oil. Arrange the onion slices on the side. Serve with breadsticks, crostini, or drape over sticks of fresh fennel.

Makes 10 servings.