Preheat the oven to 200°C (390°F).
Bring a large saucepan of water to the boil then add the cauliflower florets. Cook for 5 minutes or until just starting to become tender. Drain in a colander and set aside.
Using the same saucepan (with all the water drained away), melt the butter on low/medium heat. Add the flour and combine well, cooking over the heat for a minute to form a paste.
After a minute, add in around one third of the plant-based milk. Use a whisk to gently whisk the mixture until it comes together and thickens, making sure there are no lumps. Feel free to remove the pan from the heat at any point while you do this.
Once combined, gradually whisk in the rest of the milk and add the mustard, nutritional yeast, cider vinegar, nutmeg, salt, and pepper. Continue to whisk for a few minutes over the heat to thicken the sauce.
Once cooked, the consistency should be creamy and fairly thick. If you want it thinner, simply whisk in a little more liquid. Adjust the seasoning (I sometimes add a little more mustard) and turn off the heat.
Pour around a quarter of the sauce into a deep ovenproof dish. Add the cauliflower pieces, then cover with the rest of the sauce. If including grated vegan cheese, sprinkle this over the top.
Bake in the oven for around 20 minutes or until starting to turn golden brown. If you want some extra colour and crispiness on top, switch to the grill for a few minutes towards the end of cooking. Remove from the oven, allow to cool slightly, and serve.