Wash the green beans, trim both ends, and cut into 3-4 cm pieces.
Heat the oil in a saucepan over medium heat. Add the mustard seeds, curry leaves, and dried red chili. Let them sizzle for 30 seconds.
Add the sliced onion and minced garlic. Sauté until softened and fragrant.
Stir in the diced tomato and cook for 2 minutes.
Add the green beans, diced potato, sliced green chili, Maldive fish flakes (if using), turmeric powder, chili flakes, curry powder, and salt.
Mix everything well and cook for 2 minutes.
Cover and cook on low heat for 10-15 minutes, stirring occasionally, until the vegetables become tender.
Pour in the coconut milk and simmer for another 2 minutes.
Taste and adjust the seasoning if needed.
Serve warm with rice, roti, or other Sri Lankan curries.