Preheat the oven to 400 degrees.
For the vegetables, place the butternut squash, potatoes, parsnips, and carrots on a sheet pan and drizzle with 1/3 cup olive oil. Sprinkle with 1 tablespoon salt and 1 teaspoon pepper and toss together. Spread out in a single layer and roast for 30 minutes, turning once during cooking. After 30 minutes, toss with the garlic and roast for another 10 minutes, until all the vegetables are tender and starting to brown.
Meanwhile, line another sheet pan with aluminum foil, and place a baking rack on top of the foil. Brush the fish fillets on both sides with olive oil and season them liberally with salt and pepper. Wrap each fillet with a slice of prosciutto to form a wide band around the center of the fillet, overlapping the ends on the skin side. Arrange the fillets on the rack with the prosciutto seam side down and roast for 10 to 15 minutes, until barely cooked.
While the vegetables and fish are roasting, melt the butter over medium heat in a medium-size saute pan. Add the rosemary sprigs and cook over low heat until the rosemary leaves are crisp and the butter begins to brown, about 5 minutes. Discard the rosemary, stir in the lemon juice, and set aside.
To serve, place the fish on a platter or individual plates, spoon the rosemary butter on top, and surround with the vegetables. Garnish with lemon wedges and serve hot.
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