Transfer to the bowl of a stand mixer fitted with a paddle attachment, add the mustard and beat for 1 minute. The mixture will look like it's falling apart, but don't fret, it will come back together when you add the eggs. Add the eggs, one at a time, and beat for 2 minutes, or until fully incorporated before adding the next egg. At the end you should have a relatively thick, but pipe-able dough. If the mixture is runny or sloppy keep beating.
To make the choux pastry gnocchi add the milk and butter to a medium sized saucepan and place over a medium heat. Bring to a simmer before reducing to a low heat. Add the flour and stir continuously, with a wooden spoon, until a smooth elastic dough forms that pulls away from sides of pan.