Spicy roasted parsnip soup

INGREDIENTS

• 2 tbsp olive oil
• 1 tsp coriander seeds
• 1 tsp cumin seeds , plus extra to garnish
• ½ tsp ground turmeric
• ½ tsp mustard seeds
• 1 large onion , cut into 8 chunks
• 2 garlic cloves
• 675g parsnips , diced
• 2 plum tomatoes , quartered
• 1.2l vegetable stock
• 1 tbsp lemon juice