Heat oil in a medium pot over medium heat. Add cumin seeds and let them sizzle for 30 seconds.
Add chopped onion and sauté until golden, about 5–6 minutes.
Stir in garlic and ginger, cook for another minute until fragrant.
Add chopped tomatoes and cook until they soften and break down, about 5 minutes.
Add turmeric, coriander powder, chili powder, and salt. Stir well to combine.
Add the black turtle beans and mix to coat with spices.
Pour in water or broth. Bring to a simmer and let cook uncovered for 20–25 minutes, stirring occasionally.
Once thickened to desired consistency, stir in garam masala and adjust salt if needed.
Garnish with chopped cilantro and serve hot.