Heat oven to 180C/160C fan/gas 4. Sit the cauliflower head upside down on a chopping board and cut the tough stem away. Leave some of the base so that the cauliflower remains intact.
Heat the oil in a large flameproof casserole dish on a low-medium heat. Add the mustard seeds and, once they begin to ‘pop’, add the shallots. Fry gently for 4-5 mins until starting to caramelise, then add the mushrooms, thyme and garlic, and cook for a further 4-5 mins, stirring every so often. Pour in the wine and reduce by half. Stir in the spinach and allow the leaves to wilt.
Next, add the wheatberries, season well and combine. Make a well in the centre and put the cauliflower in the middle. Pour the stock around – not over – the caulifower, then season.
Cover and transfer to the oven. Cook for 50 mins-1 hr until the cauliflower and wheatberries are tender. Remove from the oven and sprinkle the hazelnuts, peppercorns and Parmesan over the cauliflower. Return to the oven for a further 5 mins, then serve.