Thai prawn coconut soup

Name: Thai prawn coconut soup
Prep time: 35 min
Cook time: 30 min
Total time: 1 h 5 min
Cuisine: Asian, Thai
Ingredients: 1 tablespoon vegetable oil; 2 tablespoons grated fresh root ginger; 1 stick lemongrass, minced; 2 teaspoons red curry paste; 1L chicken stock; 3 tablespoons fish sauce; 1 tablespoon light brown soft sugar; 3 (400ml) tins coconut milk; 225g fresh shiitake mushrooms, sliced; 450g prawns, peeled and deveined; 2 tablespoons fresh lime juice; salt to taste; 4 tablespoons chopped fresh coriander