To make the béchamel sauce, melt the butter in a small saucepan, then remove from the heat. Add the flour and stir with a wooden spoon to make a smooth paste. Place the pan back on the heat and stir for 1-2 minutes, or until the mixture begins to bubble. Remove from the heat and allow to cool slightly. Gradually add the milk about a tablespoonful at a time, stirring continuously to prevent lumps forming. When half the milk has been added and the mixture is smooth, add the remaining milk and stir to combine. Place back on the heat and cook, stirring continuously, until the sauce comes to the boil. Add the Dijon mustard and simmer gently for 2-3 minutes, then season well with salt and freshly ground black pepper. Next, heat the oil in a pan and quickly cook the prawns for a few minutes. Remove off the heat and place in a food processor, along with the crabmeat, chilli flakes and parsley. Season well with salt and freshly ground black pepper and add 3 tablespoons of the prepared béchamel sauce. Pulse together until combined, but still quite coarse. Place the lasagne sheets onto a clean work surface and divide the prawn mixture between the 6 sheets, placing it along the middle of each sheet. Roll up the sheets to make 6 stuffed cannelloni and place into an ovenproof gratin dish. Pour the remaining béchamel sauce over the stuffed cannelloni and sprinkle over the grated Parmesan cheese. Place in the oven to bake at 180`c for 15-20 minutes, or until golden-brown and bubbling.