Thai 'Tom Yum' Prawn Soup

Ingredients

• 12 prawns (raw, unpeeled)
• 2 stalks lemon grass (trimmed and crushed)
• 6 kaffir lime leaves
• 1-in. piece ginger (sliced in rounds)
• 2 tbsp tamarind paste
• 3 cups fresh pumpkin (peeled and diced)
• 1 cup whole oyster mushrooms
• 8 cherry tomatoes (halved)
• 2 green onions (finely sliced)
• 3 limes (juiced)
• 1 large red chili (seeded and finely chopped)
• 1/4 cup fish sauce ((optional))
• coriander and mint leaves (for garnish)