Heat the oven to 180C/160C fan/gas 4. Cut off enough of the cauliflower stalk so it sits flat, then cut a deep cross in the base of the stalk. Sit the cauliflower on a piece of foil, then rub over 1 tbsp oil. Sprinkle over 2 tbsp water, season well, then cover tightly with the foil, sealing the water and cauliflower inside the package. Put in a baking dish and cook for 1 hr 30 mins.
Meanwhile, cook the onions with a pinch of salt and 2 tbsp oil in a pan until soft and translucent. Stir in the garlic, mace, thyme leaves and flour and stir for 2 mins, then add the almond milk. Bring to a simmer, stirring, then season. When slightly thickened, remove from the heat and stir in the soya cream and mustard.
Rub the breadcrumbs with the remaining oil. Stir in the almonds.
Uncover the cauliflower and return to the dish. Pour the sauce all over the cauliflower and into the dish. Scatter the crumbs on top, and tear over the two whole thyme sprigs. Turn the oven up to 200C/180C fan/gas 6. Bake for 15-20 mins more, until the sauce is bubbling and thickened slightly, and the crumbs and cauliflower are golden. Cut into wedges and serve with a green salad and some garlic bread.