Heat the oven to 200C/180C fan/gas 6. Toss the celeriac, cauliflower florets and stems, the garlic and maple syrup with 1 tbsp of the olive oil in a roasting tin, and season well. Roast for 30-35 mins until the veg is caramelised. Remove from the oven, fish out the roasted garlic cloves, then mash them on a chopping board with a fork and set aside. Tip the rest of the roasted veg mixture into a baking dish.

Meanwhile, heat the remaining oil in a large frying pan over a medium-low heat and cook the leeks, mushrooms and thyme sprigs for 10-15 mins until the veg has softened. Turn up the heat slightly for 5 mins to caramelise the mushrooms and leeks, then carefully pour in the white wine.

Cook until the wine has almost completely reduced. Stir in the butter and, when it begins to foam, add the flour. Cook for a few minutes, stirring. Gradually pour in the milk, stirring well after each addition. Bring to a gentle simmer and cook for 5 mins, stirring often until thickened. Remove from the heat, add the mashed roasted garlic and the cheddar, and stir until the cheese has melted. Season well. Roughly chop the cauliflower leaves and stir these through the sauce.

Pour the cheese sauce into the baking dish with the roasted veg, and stir to combine. Sprinkle over the sourdough crumbs, then return to the oven for 15-20 mins, or until the top is golden and the sauce is bubbling at the edges. Serve alongside roast chicken, or as a main with a green salad on the side. To freeze, leave to cool completely in the dish, wrap well and freeze for up to three months. Defrost completely in the fridge overnight, then bake as directed.