In a large saucepan over medium-high heat, heat the oil. When the oil is hot, add the onion, thyme, ginger, garlic, and curry leaves. Sauté for 2 minutes, then add salt to taste and cook until the onion softens, about 5 minutes more. Add the tomatoes and ½ cup (120 ml) water. Bring to a simmer and then turn the heat to low, cover, and simmer for 15 minutes.