lime och citrongräskryddad potatis- och rotsellerisoppa med purjo, äpple och päron

___You need:
1 tablespoon unsalted butter
1 tablespoon olive oil
1/2 red onion, peeled and chopped
1 leek, white part only, chopped
5 to 6 twigs of lemon thyme
14 oz (400 g) peeled and diced celeriac
10.5 oz (300 g) peeled white sweet potato
1 Bosc pear, peeled, cored and diced
2 Heirloom apples, peeled, cored and diced
1 lemongrass stick, diced
1 kaffir lime leaf
4 cups cold water Sea salt and pepper__

In a pot, melt the butter. Add the olive oil and when warm, add the onion, leek and lemon thyme. Cook, stirring occasionally, until the onion and leek soften (about 4 minutes).
Add the rest of the vegetable, the apples and pear, and cook for 5 more minutes.Add the water, kaffir lime leaf, lemongrass and season with salt and pepper. Cover and bring to a boil. Simmer for 20 minutes, or until the vegetables and fruit are soft.
Discard the lemongrass, kaffir lime leaf and lemon thyme and transfer the soup to the bowl of a food processor. Purée finely. Serve the soup warm with bay scallops lightly sauteed in olive oil. Decorate with fish roe and fresh herbs.