Season the fish with salt and pepper. Arrange the shrimp on the fish. Roll up to enclose the shrimp. Place seam side down close together in a shallow baking dish. Sauté the leeks in butter in a skillet until softened. Spoon over the fish. Whisk the cream, tomato paste, Dijon mustard, cornstarch and salt and pepper in a bowl. Pour over the leeks and fish. Bake at 350 degrees for 30 minutes.
Note: This dish can be prepared and refrigerated before cooking.