In a large skillet or sauté pan, sauté the onions gently in olive oil until they are quite soft and translucent, seasoning the onions with salt and pepper as you go.
Add the canned tomatoes, crushing them with your hands as you add them to the pan. (Or pass them through a food mill if you don't like getting your hands messy...) Let the tomato simmer for 10-15 minutes, until the tomatoes have reduced down to a saucy consistency.
Add the baccalà pieces, along with the raisins and pine nuts. Season with salt and pepper. Turning the pieces in the sauce so they are covered all over, adding a bit of water to loosen the sauce if needed. Cover, leaving the lid slightly ajar, for a good 20-30 minutes, and simmer gently until the baccalà is cooked through and the sauce has thickened nicely. Check from time to time, and things are drying out, add a tad more water. Before serving, taste and adjust for seasoning.
Serve still hot, topped if you like with some minced parsley for color.