Saute the onion garlic and mustard seed in oil until the onion softens.
Stir in the chickpeas, tomato sauce, vegetable broth, and spices.
Add meatballs to the curry sauce and bring to a boil; reduce heat and simmer for 15 minutes, until sauce thickens; remove from heat and let stand five minutes.
In a small bowl, whisk the yogurt with water, then stir into slightly cooled curry.
Serve curry with steamed rice or flat breads, as you prefer.