Start the oven on 220 degrees Celsius on electrical heat.
Peel the shrimps or the crayfish.
Chop the shrimps/crayfish and the fish in pieces.
Melt the butter in a sauce pan, and add the flour in the melted butter and stir it together.
Add the milk in the butter, and stir all the time until the milk start to boil and there is no flour lumps.
Lower the temperature for the sauce pan and add half of the grates cheese. Add the salt, black pepper and the dill in the sauce pan and continue stirring the sauce until the cheese melts.
Put a layer of lasagna plate in a oven form, add half of the shrimps/crayfish and the fish, and then add 1/3 of the sauce on top of that. Do the same with another layer of lasagna plates, the rest of the shrimps/crayfish an the fish and half of the sauce. Put another of layer of lasagna plates and add the rest of the sauce on top. Sprinkle the rest of the grated cheese on top of the sauce.
Put the lasagna in the middle of the oven for 30-40 minutes.
It is very good to serve fresh vegetables to the lasagna.