Heat oven to 400F. Season the fish all over, then wrap each fillet in a slice of prosciutto. Put into a large baking dish. Dot the crème fraîche between the fillets and over the exposed ends of the fish. Dot the pesto around the fish, too. Scatter with the cheese.
Bake the fish for 15-20 mins, adding the pine nuts halfway through, until the crème fraîche has made a sauce around the fish, and the cheese and prosciutto are turning golden. Serve with plenty of crusty bread to mop up the sauce.