Rinse fillets in cold water, pat dry and season with salt and pepper. Combine dry stuffing 110g melted butter and just enough hot water to make the mix come together.
Cover a baking tray with foil. Spread stuffing on top of foil. Place fillets on top of stuffing. Brush fillets with remaining butter and sprinkle with onions. Top with dill and cover the baking tray tightly with another piece of foil.
Bake on centre shelf of preheated oven for 10 minutes, reduce oven temperature to 200 C / Gas 6 and bake for an additional 60 minutes.