Cut potatoes into 3cm cubes. Cook in boiling salted water for 8-12 minutes until tender. Heat oil in a wok over medium-high heat. Cook onion with 1 teaspoon salt for 1-2 minutes to soften. Add paste and cook for 1 minute until fragrant. Add coconut milk and cook for 5 minutes. Add stock, sauce, juice, sugar, spring onion and fish. Cook for 3 minutes or until fish is cooked, then add potatoes and chopped coriander. Serve with rice, extra coriander and chilli.