Heat oil in a wide pan on medium heat. Add cumin seeds. When they sizzle, add hing if using.
Saute onion until soft and lightly golden, 6 to 8 minutes.
Add ginger, garlic if using, and green chilies. Stir 20 to 30 seconds.
Add potatoes, turmeric, part of the salt, coriander powder, red chili powder, and Kashmiri chili if using. Stir to coat. Cover and cook on low-medium heat 6 to 8 minutes, stirring once, until potatoes are partly tender. Add 1 tbsp water only if the pan scorches.
Add tomatoes. Cook until they break down and oil separates, 5 to 7 minutes.
Add okra. Stir gently, spread in a layer. Cover and cook on medium heat 4 minutes.
Uncover. Cook on medium-high 6 to 10 minutes, stirring only a few times, until okra and potatoes are tender and the sabzi is dry.
Turn off heat. Add amchur or lemon juice, garam masala, and remaining salt if needed. Garnish with coriander.