Heat oil in a heavy-bottomed kadai or pan over medium heat. Add cumin seeds and let them sizzle for 30 seconds until fragrant.
Add the chopped onion and cook on medium-low heat for 6-8 minutes, stirring occasionally, until soft and translucent. Do not rush this step — properly cooked onions are the foundation of the masala.
Add the grated ginger and slit green chili. Stir and cook for 1 minute.
Add the chopped tomatoes, turmeric, coriander powder, cumin powder, red chili powder, and salt. Mix well. Cook on medium heat for 8-10 minutes, stirring occasionally, until the tomatoes break down completely and the oil starts to separate at the edges. The masala should look thick, not watery.
Add the diced lauki. Toss to coat in the masala. Reduce heat to low-medium, cover the pan, and cook for 10-12 minutes, stirring every 3-4 minutes, until the lauki is tender. A fork should slide in easily.
Taste and adjust salt. Garnish with fresh coriander leaves. Serve hot.