Place the shallots and 10g of butter in a saucepan and sweat for about a minute, making sure that the butter doesn't burn. Add prawns, crabmeat and cognac. Season to taste.
Cook for 2-3 minutes.
To make the béchamel sauce, place butter and flour in a pan and mix together over medium heat. Add the milk slowly to turn it into smooth roux sauce. This sauce should have a rather thick consistency. Add half of the béchamel sauce to the prawn and crab mixture and sprinkle with fresh dill. Have the pasta rectangles ready on a work surface in front of you. Fill them with the mixture and then roll into tube shapes. Butter an oven dish and arrange the cannelloni in it. Cover with the rest of the béchamel sauce and sprinkle with the Parmesan cheese. Add few knobs of butter if you wish. Place in preheated oven for about 8 minutes or until surface is well browned. Serve with crispy green salad.