Four-bean curry

Name: Four-bean curry
Cuisine: Asian, Indian
Ingredients: 2 T oil; 1 onion, chopped; 1 t ginger, minced; 1 t garlic, minced; 2 t curry powder; ½ t ground turmeric; 2 tomatoes, grated; 1 t sugar; 1 cup water; sea salt and freshly ground black pepper, to taste; ¼ cup coconut milk; 200 g green beans, blanched; 1 x 400 g can butter beans, drained; 1 x 400 g can kidney beans, drained; 1 x 400 g can black beans, drained; coriander, to garnish; rice or flatbread, for serving