Heat the oil in a large nonstick wok or frying pan over a medium-high heat. Add the cumin seeds, curry powder and green chilli and stir-fry for 1–2 minutes until fragrant.
Add the ground spices, tomatoes, potatoes and green beans. Season to taste and stir-fry briskly over a high heat for 4–5 minutes. Remove from the heat and stir in the mint. Squeeze over the lime juice and serve immediately.