Pour the olive oil into a large pot over medium-high heat. When hot, stir in the onion, and garlic. Sauté for a couple minutes, just long enough for things to soften up. Stir in the curry paste, potatoes, cauliflower, and salt, and allow to cook for another 5 minutes or so. Add 4 1/2 cups of water, and bring just to a boil. Dial back the heat and simmer just long enough for the cauliflower and potatoes to get tender throughout, another 5-10 minutes.
