Brined roast chicken with roasted sweet potatoes and tahini yogurt

Ingredients

• 150g sea salt
• 25g castor sugar
• 10 black peppercorns
• small bunch of thyme
• 1 large chicken, about 1.8-2kg
• 2 lemons
• 1 head of garlic, 2 cloves crushed for the sauce and the rest left unpeeled
• 800g sweet potatoes, peeled and cut into thick chunks
• 2tbsp olive oil
• 200g plain yoghurt
• 3tbsp tahini
• 5tbsp flat-leaf parsley, finely chopped