Rinse okra, dry thoroughly with towels, then slice into 1/2-inch pieces.
Warm oil in a large skillet over medium heat. Sprinkle in cumin seeds and let them sizzle for 30 seconds.
Add sliced onion and sauté until soft and lightly golden, about 5 minutes. Stir in green chili if using.
Stir in garlic and ginger. Cook until fragrant, 30–60 seconds.
Add tomatoes, coriander, turmeric, chili powder, and salt. Cook until thick and jammy, 3–4 minutes.
Toss in okra and spread it out. Cook uncovered, stirring gently, until tender and non-slimy, 8–10 minutes.
Finish with garam masala and lemon juice. Taste and adjust salt or heat.
Turn off heat and scatter cilantro on top. Serve hot.