Clean herrings, bone and divide into fillets. Spread the insides with the mustard, sprinkle with salt and pepper and roll up. Peel and slice potatoes and onion; peel, core and slice apple. Put half the potatoes into a greased pie-dish, add apples, then onion and the herring rolls. Sprinkle with the sage. Cover with remainder of apple and potatoes and seasoning and half fill the dish with boiling water. Put little nuts of margarine on top, cover, and bake in a moderate oven for an hour; remove cover and leave in the oven for another half hour.