Trim all of the dark meat from the fillets. Rinse in cold water, dry and sprinkle with salt and pepper. Combine breadcrumbs with boiling water and 125g melted butter.
Cover a baking tray with a lightly greased double thickness of aluminium foil. Spread stuffing mixture on top of foil. Place fillets on top of stuffing. Brush fillets with 2 tablespoons melted butter and sprinkle with onions. Top with dill springs and crimp together edges of tin aluminium foil to seal.
Bake on centre self of preheated oven for 10 minutes. Reduce oven temperature to 200 degrees C and bake for an additional 60 minutes.