Heat the oil in a large, heavy soup pot over medium heat and add the onion. Cook, stirring, until tender, about 5 minutes, and add the garlic, ½ teaspoon salt, the cumin, coriander, and curry powder. Stir together for about a minute, until the garlic is fragrant, and stir in the tomatoes with their juice. Bring to a simmer and cook, stirring often, for 10 minutes, until the tomatoes have cooked down slightly. Add salt to taste.
