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Heat the oven to 200C/180C fan/gas 6. Toss the cauliflower florets and stalks with the oil in a large roasting tin and season well. Roast for 25-30 mins until almost tender, then add the leaves and cook for a further 5 mins.
To make the sauce, melt the vegan butter block in a large saucepan. Add the flour, mustard powder, onion and garlic granules and cook for 2-3 mins. Gradually add the soya milk, whisking well to incorporate. Bring to a gentle simmer, stirring constantly, then simmer for 4-5 mins until thickened and reduced slightly. Add the nutritional yeast and season well, then whisk to combine.
Tip the sourdough breadcrumbs and vegan cheese into a bowl and stir to combine. Season well. Remove the cauliflower from the oven, spoon over the sauce and stir to combine. Scatter over the cheesy sourdough crumbs and return to the oven for 10 mins until bubbling and golden. Serve as a side or main with a green salad.