In a 10- to 12-inch frying pan over medium-high heat, stir onion, ginger, and garlic in oil until onion just begins to brown, 5 to 8 minutes. Add curry powder, cumin seeds, mustard seeds, and cayenne and stir until fragrant, about 30 seconds.
Add garbanzos and tomatoes (including juice). Bring to a boil, reduce heat, and simmer uncovered, stirring occasionally, to blend flavors, about 5 minutes. If mixture is thicker than desired, stir in a little water to thin. Add salt to taste. Spoon into serving dishes.
Serve with yogurt on the side.