Melt butter in a medium saucepan over low heat. Add mushrooms, shallots, salt, and pepper, and cook until soft, about 10 minutes. Preheat oven to 450 degrees F.
Raise heat to high, add Port, and reduce by half. Add clam juice or fish stock and reduce again by half. Add cream and reduce one more time by half. Remove from heat, strain sauce, and stir in peppers and olives. Reserve in a warm place.
Season fillets all over with salt and pepper. Melt remaining 4 tablespoons butter in a large skillet over high heat. Saute fish for 1 minute. Turn over, transfer to oven, and bake, uncovered, for 3 to 5 minutes. Arrange fish on individual plates. Spoon on warm sauce and garnish with julienned prosciutto. Serve immediately.