Put the sweet potatoes in a food processor and pulse until the largest pieces are about 1/2 inch and the smallest about 1/4 inch. Set aside.
In a large, deep, nonreactive skillet, heat the olive oil. Add the scallions and cook over moderately high heat until just softened, about 3 minutes. Add the garlic and cook until fragrant, about 1 minute. Pour in the white wine, bring to a boil and boil for 1 minute. Add the sweet potatoes, tomatoes, cardmom, 1/2 tablespoon salt and 1/8 teaspoon pepper. Reduce the heat to moderate, cover tightly and cook until the potatoes are just tender, about 20 minutes.
Using tweezers or pliers, remove the small pin bones that run the length of the bluefish fillets. Season the bluefish with salt and pepper and arrange the fillets, skin side down, on top of the vegetable ragout, overlapping them slightly if necessary. Sprinkle the bluefish with the parsley, cover tightly and cook until the fish begins to flake, about 15 minutes. Transfer the bluefish fillets and the vegetable ragout onto serving plates, spoon some of the cooking juices over the fish and serve.
The vegetable ragout can be refrigerated for up to 3 hours.
An oak-aged Chardonnay has enough edge to provide contrast and enough rich flavor to underscore the fish and ragout. Consider one from Napa Valley.