How to Make It
In a small skillet, toast the fennel seeds over moderate heat until fragrant, about 1 minute. Let cool.
On a rimmed baking sheet, spread the onion slices in the same shape as the fish fillet. Top the onions with most of the fennel fronds, reserving a few for garnish. In a small bowl, mix the salt with the sugar and fennel seeds and sprinkle over the fennel fronds. Set the fish on the curing mixture, skin side up, and pour the grappa over the fillet. Cover with plastic wrap. Set a baking sheet on top of the fish and weigh it down with heavy cans. Refrigerate for 24 to 36 hours.
Unwrap the fish. Drain and scrape off the curing mixture; transfer the fish to a cutting board, skin side down. Using a thin-bladed knife and starting at the tail end, thinly slice the fish on the diagonal. Avoid the darker flesh near the skin. Serve the slices on toasts drizzled with olive oil and sprinkled lightly with coarse sea salt and fennel fronds.