Shrimp soup with lemongrass and sweet potato

For the broth

½ onion ½ garlic clove 1 lemongrass stalk (halved) 2 tbsp olive oil Heads and shells from 12 large shrimp ½ liter vegetable stock or water

For the soup

1 sweet potato (200 g) 1 tbsp olive oil ½ onion 1 zucchini Juice from ½ lemon Salt Pepper

For serving

Fresh coriander