Melt the vegan Butter in a pan, over medium heat. Once the butter has melted, add the flour. Using a wooden spoon, stir quite vigorously to make a smooth glossy paste or a blond roux, as we call it (see notes below). You will know it's done when the mixture is a light beige colour.
Take off the heat and add the Oat milk a little at a time. Whisk constantly making sure no lumps form. By adding a little milk at a time and whisking, using a baloon whisk, you will achieve a smooth, glossy, creamy sauce.
Once all the milk has been incorporated, bring the pan back to a medium-low heat and cook further for about 5 minutes or until the sauce has reached coating/nappe consistency (see notes below). Whisk from time to time, to make sure nothing sticks to the bottom of the pan.
Add the mustard, nutmeg, salt & pepper and half of the vegan cheese. Set aside.
Parboil (see notes below) the Macaroni until it begins to turn soft. Drain immediately and cool with cold water (omit this process of cooling if you are mixing in the cheese sauce and are going to bake straight away )Set aside.
Preheat the oven to 180ºC
Mix the Macaroni into the cheese sauce, transfer to a baking dish, and add a layer of grated vegan cheese.
Pulse the peas in a blender so that they are a bit mushy. You don't want a puree, just the peas broken down a little. Add this to the top of the Mac & cheese. Grate some vegan Parmesan over the pea crust.
Heat the oven to 180ºC and bake the Mac & Cheese for about 20 minutes, until golden brown on top.