Peel and thinly slice the jerusalem artichokes. Melt the butter and olive oil together in a pan. Fry the fish fillets 3 minutes on each side. Set aside, keep warm. To the same pan add the wine, scraping the bottom of the pan. Add rosemary, lemon juice, jerusalem artichokes, heavy cream, salt and pepper. Cook until the jerusalem artichokes soften and the sauce is reduced to a half, about 1 to 1 1/2 hour. Transfer the fish fillets to serving plates, pour the sauce over the fish, and garnish with rosemary sprigs.