Pat the halibut fillets dry with a paper towel and season with salt & pepper. Grease the bottom of an oven proof sauce pan with 1 tbsp of the butter and place the seasoned fillets in it. Sprinkle fillets evenly with the sage, pine nuts and the prosciutto. Carefully pour the fish stock around the halibut and bake at 350 degrees for 8-10 minutes, or until the fillets are cooked through.
When finished cooking, remove the fish from the pan to a warm serving dish and cover to keep warm. Put the pan on the stove at medium high heat. (If there isn’t at least 1 cup of liquid in the pan, add some more fish stock.) Stir in the last 2 tbsp of the butter and the lemon juice. Cook for 4-5 minutes, stirring occasionally, until the sauce has reduced by half.
Pour the sauce over the halibut and serve immediately.