For this dish I would recommend buying prepared and cleaned crabmeat as otherwise this could take some time to cook and clean the crab. You will still need to go through the crabmeat in case of bones and pick them out and then dry on a paper towel.
Start by preparing the white sauce. Melt the butter in a pan on a low heat then add the flour and cook out for 5 minutes stirring regularly. While this is cooking, place the milk in to a pan and bring to a simmer. Pour the milk little by little into the flour and butter mix stirring all the time and have the heat on low so it is not hot as it will burn on the bottom of the pan. Add the salt and pepper, simmer for 8 minutes and slow temperature.
Continue with the grain mustard sauce by melting the butter in a pan on a low heat, add the onion and cook on a low heat for 5 minutes so they sweat and do not colour. Season lightly, add the wine and reduce by 2/3; add the white sauce, mustard, spring onion and cream and simmer for 2 minutes stirring all the time and then turn off the heat.
Weigh all the ingredients for the parsley crumb topping then place the crumbs, garlic, olive oil, lemon zest, salt, pepper into a robo coupe and blitz until the crumbs are coarsely ground, then place in a bowl and add the parmesan cheese.
To prepare the pie, mix the sauce with the crabmeat and place into a suitable pie dish. Sprinkle over the crumb topping and some grated gruyere cheese on top of this. Bake in a 180°C oven for 25 minutes until the crumb is melted and golden in colour.