Cook the onions in a large saucepan in 2-3 tablespoons of olive oil until softened and beginning to color, about 20 minutes. Add the reserved sardine heads and bones, the wine and the squid ink. Bring to a boil and boil for 10 minutes. Strain the sauce through a fine-mesh strainer into a large saucepan. Return the saucepan to the heat and add the lentils. Bring to a boil, reduce heat and simmer for 25 minutes. Cool completely. Refrigerate for 24 hours. Place the sardine fillets in a shallow earthenware dish and pour in the sherry vinegar. Cut the small onions in half and add them to the sardines. Cover and let stand for 2 hours. To serve, drain the lentils and place a large spoonful of them on each plate. Place 3 sardines on each bed of lentils. Garnsh with the button onions. Spoon over a little of the marinade juice. Serve as a starter.
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1 1/4 cups lentils, washed
18 fresh sardines, cut into fillets heads and bones reserved
