To make the sauce, melt the butter in a frying pan over a medium heat and cook the shallots with a large pinch of salt for 5-10 minutes or until softened. Pour in the wine and reduce by half, then pour in the cream and reduce again until saucy. Stir in the mustard, then flake in the salmon and continue to cook for a few minutes to warm through. When ready to serve, gently stir in the salmon roe (if using), lemon juice and chives, then season. Spoon the filling into the vol-au-vent cases, top with the lids, if you like, and serve.