Celeriac and Tahini Gratin | Marcus Wareing | Marcus Wareing

Ingredients

• butter, for greasing
• 800g floury potatoes, peeled
• 1 large celeriac, peeled
• 2 white onions, thinly sliced
• 4 sprigs of thyme, leaves picked
• 200g tahini
• 600ml good-quality vegetable stock, warmed
• ½ tsp fine sea salt
• ½ tsp ground white pepper
• 1 tsp garlic powder
• handful of grated mature Cheddar cheese, to cover the top
• freshly ground black pepper