Combine currants and 1/3 cup wine in a glass measuring cup, microwave on high 1 minute, and let steep while preparing the casserole. Season swordfish with salt and pepper, and dredge in flour; pat off excess flour.
Pour 1 1/2 tablespoons oil into a large skillet, and heat over medium-high heat. Brown steaks 1 1/2 minutes per side, until golden (but not cooked through). Transfer to a shallow 2 1/2- to 3-quart casserole or baking dish. Pour remaining 1/3 cup wine around fish.
Heat remaining 1 1/2 tablespoons oil in same skillet over medium heat; add onion, and cook 5 minutes or until translucent. Add the soaked currants (and any wine left in the cup), and stir in tomatoes and next 4 ingredients. Continue to cook 7 minutes, stirring occasionally, until squash is tender and flavors have blended. Top each steak with vegetable mixture; sprinkle with pine nuts.
Bake at 400° for 12 minutes or until fish is done. Serve immediately, spooning any pan juices over portions. Top with basil leaves.
Casserole Basics: Glass and ceramic (earthenware) dishes work best for casserole cooking because they heat up quickly and evenly. If you substitute a metal baking pan, the casserole usually cooks more slowly, so you may have to increase the oven temperature by 25° to compensate.
Always start with the freshest seafood available and pat it dry. If you use individual steaks or fillets, select pieces that are at least 1 inch thick. If a single fillet from a large fish tapers to a thin end, fold the thin end under to help the fillet cook uniformly.
If the fillet in the casserole is thick, let it sit at room temperature while you prepare the other ingredients for the dish. It will cook more quickly.
Thick fillets or steaks, especially if covered with a sauce or topping, can take from 12 to 15 minutes of baking time per inch. To check for doneness, cut into the fillet at the thickest part to see if it's opaque inside and pulls apart with little resistance.
Casserole recipes calling for fully or partially cooked seafood or fish need only enough time in the oven to get hot, so an instant-read thermometer comes in handy. If the middle of the casserole registers 160°, it's done.
If you're making a casserole with cooked seafood that's added to a sauce, heat the sauce separately, stir in cold, cooked fish, and then assemble the dish. This will speed up the time in the oven and keep the fish from disintegrating.
Don't cook fish until it flakes easily-- by that point, the fish has given up all of its juices, rendering it dry and leaving a lot of water in the dish. If this occurs, spoon the juices back over the fish when serving.
Remember, the casserole will continue cooking once it's pulled from the oven, so it's better to take it out sooner rather than later.