In a large pan or kadhai, heat the mustard oil over medium heat until slightly smoky.
Temper the hot oil with whole dried chilies, hing and cumin seeds. Let crackle.
Add the chopped onions and sauté until they start turning golden brown. This takes about 5-6 minutes.
Reduce the heat to low and add the garlic paste and all spice powders(except garam masala and amchur). Add 1-2 tablespoon of water to make sure that the spices don't burn. Fry the garlic and spices for another 1-2 minutes until the raw smell disappears.
Toss in the potato cubes in the masala and sprinkle salt. Since potatoes take longer to cook as compared to green beans, we are adding them earlier to get them started. Fry the potatoes for 1-2 minutes with masala, stirring occasionally to prevent sticking.
Then add the chopped tomatoes and mix. Cover pan and let cook for 2-3 minutes until they soften and start to break down. You will see that masala becomes runny because of tomato juices. Don't dry out too much.
Uncover and add the green beans, mix and sprinkle the roasted besan (if using). At this point, I add little water (about ? cup water) so that the sabzi isn't too dry. Taste and adjust the salt at this stage.
Cover the pan again. Cook for another 6-8 minutes, stirring occasionally, until the green beans and potatoes are tender.Adjust time as needed.
Once the vegetables are cooked through, sprinkle garam masala and amchur powder (or lemon juice) over the sabzi. Give it a final stir, switch off the stove. Let sit uncovered for few minutes to make sure that vegetables don't overcook.
Serving - Garnish with freshly chopped cilantro leaves and serve hot with plain parathas, tawa roti, tandoori roti, or dal-chawal (rice). You can add a bowl of raita on side.