Cooking fish with vegetables is common in the Eastern regions of India. You can add lots of vegetables and use okra to thicken the sauce.
Heat oil or ghee in a pan over medium heat. When the oil is hot, toast the cumin seeds until fragrant, about 30 seconds. Add the onion, pepper, garlic, chili, and salt. Saute until onion is soft, about 5 minutes. Stir in the ground cumin and coriander and saute another minute or two. Add the tomatoes, potatoes, and stock. Bring to a boil and simmer for 10 minutes. Add fish chunks and simmer another 5-6 minutes. Add greens at the end, just long enough for them to brighten in color, but not to overcook.