In a frying pan over medium-high heat, warm the oil. Add the onions and garlic and sauté until softened, about 5 minutes. Add the ginger, coriander, turmeric, and cumin seeds and sauté until fragrant, about 1 minute. Add 1cup (8fl oz/250ml) hot water and deglaze the pan, stirring to scrape up the browned bits on the pan bottom. When the water comes to a boil, remove from the heat.
Cook the curry
Put the potatoes, cauliflower, and green beans in a Dutch oven or slow cooker and add the contents of the frying pan. Sprinkle with 1½teaspoons salt and stir to combine. If using a Dutch oven, partially cover and cook over low heat until the vegetables are tender and the sauce is thick, about 30 minutes. (If using a slow cooker, cover and cook on the high-heat setting for 4 hours or the low-heat setting for 8 hours.) Stir in the cilantro, season to taste with salt, and serve.