Advance work: Heat 4 tablespoons olive oil in a heavy skillet; add the onions and stir. Cover the pan and cook over low heat for 15 minutes, stirring once or twice. Uncover the pan, raise the heat, and reduce the liquid produced by the onions. Lower the heat, return the cover, and cook for another 15 minutes. Repeat this process until the onions are a light caramel color. Four 15- minute periods should do it. Do not scorch.
While the onions are reducing, steam the shellfish in the wine until they open, and reserve in the shells for a splendid presentation. Reserve the broth; you’ll have about 1 cup.
Cook the garlic briefly in the reduced onions. Add the rest of the ingredients, except the parsley, flour and swordfish. Cover and simmer for about 15 minutes, then uncover and reduce to a thick sauce. The recipe may be prepared in advance to this point.
At dinnertime, heat the sauce and shellfish together. Lightly flour the swordfish slices, and brown them briefly in olive oil on both sides. Put the fish pieces on a serving platter, top them with the sauce and shellfish and serve sprinkled with parsley.
The Wine List Recommendation: Vinho verde literally translates to “green wine”—but is more a reflection of the wine’s crisp, fresh qualities rather than its color. A good one to try is Jose Maria da Fonseca “Twin Vines” inho Verde, Portugal which retails for about $10.00. If you can’t find vinho verde (often only available in the summertime), a youthful Spanish white would also marry perfectly with the garlicky and slightly spicy notes in the sauce. Try: Sitios de Bodega Con Class (Sauvignon Blanc, Viura, Verdejo), Rueda, Spain $12.00.