1/4 cup cooking oil
1 onion, thinly sliced
6 zucchini - peeled, seeded and cut into semicircles
1/2 teaspoon salt
2 cups water
2 teaspoons chili powder
1/2 teaspoon ground turmeric
1/2 teaspoon garlic powder
1 teaspoon ground coriander seed
3 whole cloves
7 whole peppercorns
4 tomatoes, chopped
2 tablespoons plain yogurt