Oyster Blade of Beef With Celeriac and Bone Marrow

Ingredients

• 1.6 kg piece oyster blade
• 2 tbsp olive oil, plus extra for drizzling
• 3 litres veal stock
• 100 ml good-quality balsamic vinegar, plus extra for seasoning
• 50 gm dried porcini mushrooms
• 1 each thyme sprig and rosemary sprig
• 20 gm finely grated horseradish (¼ cup)
• To serve: wood sorrel (optional; see note)
• Baked celeriac
• 500 gm celeriac (about 1 large), cut into 8 wedges
• 2 tbsp olive oil
• 3 thyme sprigs, leaves picked
• Celeriac purée
• 500 gm celeriac (about 1 large), diced
• 250 ml milk (1 cup)
• 50 gm butter
• Bone-marrow crust
• 100 gm chestnut mushrooms, trimmed
• 100 gm small Swiss brown mushrooms, quartered
• 1½ tbsp olive oil
• 100 gm diced bone marrow (see note)
• 80 gm pickled walnuts (about 3; see note), coarsely chopped
• 30 gm unsalted butter
• 30 gm panko crumbs