Asian-style baked onions, potatoes & sweet potatoes - Good Food Middle East

Ingredients

• 1 tsp coriander seeds
• 1 tsp Sichuan pepper (or dry green or black peppercorns)
• 1 tsp kalonji black onion seed (aka nigella)
• 2 'flowers' star anise
• a large pinch sea salt
• 2 large Spanish onions, peeled
• 3 large, not-too-floury potatoes, such as Desirée, unpeeled
• 2 large sweet potatoes, peeled
• 1 tbsp groundnut oil or vegetable oil
• 2 tbsp tamari or light soy sauce (tamari is a thick soy sauce made with rice)
• fresh lime or lemon juice to serve (optional)