Photograph: Brett Stevens
Preheat the oven to 220°C, fan 200°C, gas 7. Trim the shoots of the beetroot to about 1cm and the whiskery tail to about 5cm, and give them a good scrub. Put in a roasting tin and drizzle with the oil. Cover with kitchen foil and roast for 50 minutes.
Pull off half the thyme leaves from the sprigs. Remove the foil then scatter the thyme leaves and sprigs over the beetroot. Add the lemon juice, garlic and some seasoning and toss everything together. Return to the oven, uncovered, for a further 5-7 minutes until the beetroot is tender. Leave to cool.
Meanwhile, to make the hazelnut butter, toast the hazelnuts for 4 minutes in the same oven as the beetroot. Leave to cool. Whiz to a coarse meal in a food processor, then add the butter, a little cayenne pepper and sea salt and whiz again.
In the roasting tin, halve the beetroot through the stems and turn to coat in the oil. Place them cut-side up in a clean ovenproof dish that holds them snugly. Dot with the hazelnut butter.
To finish, preheat the grill. When hot, pop the beetroot underneath the grill until the nut butter is golden and sizzling. Meanwhile, toss the watercress with the nut oil and a pinch of salt. Serve the beetroot with the dressed watercress and lemon wedges.
Get ahead: cook the beetroot and make the hazelnut butter the day before; cover and chill. Bring to room temperature before finishing the recipe.