Carefully remove eight large outer leaves from the Savoy cabbage. Blanch for about 2 minutes until flexible but still vibrant green, transfer to iced water, drain and pat dry. Carefully shave down the thick central rib without cutting through the leaf.

Cook shallots in butter and olive oil for about 3 minutes. Add garlic and thyme, then the mushrooms. Cook until the moisture evaporates and the mixture is almost dry. Stir in chestnuts, Dijon mustard, Parmesan and parsley. Finish with lemon juice, salt and black pepper. Cool completely.

Lay two overlapping cabbage leaves on the work surface. Place one-quarter of the mushroom filling in the centre. Fold the sides inward and roll tightly. Repeat to make four parcels.

Preheat the oven to 190°C/170°C fan. Arrange parcels seam-side down in a baking dish. Add stock, butter and thyme. Cover with foil and bake for 15 minutes. Remove foil, baste and roast uncovered for a further 8–10 minutes until glossy with lightly caramelised edges.

Cook potatoes in salted water until tender. Drain and steam dry. Pass through a ricer. Warm cream and milk, then beat into the potatoes. Add butter gradually until smooth and glossy. Finish with truffle oil, salt and white pepper.

Cook onions slowly in butter and olive oil for about 20 minutes until deeply caramelised. Add brown sugar, then balsamic vinegar. Add Worcestershire sauce, vegetable stock and thyme. Simmer until reduced by half, blend, strain and reduce until glossy. Whisk in cold butter before serving.

Cook mushrooms over high heat in olive oil until deeply golden. Add butter, garlic and thyme and baste briefly. Season to taste.

Fry sage leaves in neutral oil for 15–20 seconds until crisp. Drain and season lightly.

Spoon truffle potato purée onto warm plates. Place one cabbage parcel partly over the purée and cut diagonally if desired to reveal the filling. Arrange roasted mushrooms around it, spoon caramelised onion jus around the plate and finish with crispy sage, thyme, truffle oil and black pepper.