Step 1Heat one-fourth to one-half inch of canola oil in a heavy skillet until it is hot enough that food sizzles when added. Lightly flour the sardines and fry until lightly browned, about a minute per side. Lift the sardines from the oil and drain them on paper towels. Season with salt.
Step 2Drain the oil from the skillet, but don't wipe it clean. Add the onions and the olive oil and cook over very low heat until the onions wilt and turn golden (not brown). This can take as long as an hour. Stir the onions from time to time.
Step 3When the onions are soft and sweet, add the vinegar, wine and bay leaf and increase the heat to medium. Cook until the liquid has reduced to a glaze, about 10 minutes. Remove the pan from the heat, discard the bay leaf, and stir in the raisins and pine nuts. Cool.
Step 4Arrange a layer of sardines skin-side up in the bottom of a small (10 1/2-inch by 6 1/2-inch) baking dish. Cover with half the onions. Repeat, layering the remaining sardines and onions. Pour any liquid that's left in the pan over the top. Cover with plastic wrap and refrigerate at least 2 days before serving.
Step 5To serve, spoon the sardines and onions onto toast.