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.
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.
Bring a large saucepan of water to a boil and season with salt. Fill a disposable piping bag (or a piping bag fitted with a 2cm nozzle) with the choux mixture and cut the disposable piping bag so that you have an opening of about 2cm. Pipe the mixture into the boiling water in 2 batches, and cook until the gnocchi float to the surface. Remove the gnocchi with a slotted spoon and drain on paper towel. Place the gnocchi in the refrigerator for at least an hour, uncovered, to dry.
For the mornay melt the butter in a medium sized saucepan over medium heat before adding the flour. Cook the flour and butter mixture, stirring, for 1 to 2 minutes, or until the mixture starts to bubble. Gradually add the wine and whisk until smooth before gradually adding the milk. Whisk until smooth and cook until the sauce coats the back of a spoon. Remove from the heat and add the cheese and nutmeg. Stir to combine and check for seasoning and adjust as necessary. If not using immediately cover with plastic wrap, with the plastic touching the surface of the sauce to prevent a skin forming.
When ready to serve, place the mornay over a medium low heat to warm and place a large non stick frying pan over a high heat and add butter and oil and mushrooms and season. Cook for 2 to 3 minutes before adding the rosemary. Cook for 1 further minute and place the mushrooms in a bowl. Return the frying pan to a high heat and add the vegetable oil. The pan should be smoking hot before adding the gnocchi. Add the gnocchi and shake the pan to cook the gnocchi on all sides and prevent sticking. When cooked divide between serving bowls and top with the mushrooms and mornay sauce.
The pan needs to be smoking hot when frying the gnocchi otherwise the gnocchi will stick. If they do stick, don't stress. Just cover them in the mornay sauce and no-one will ever know!