If using lamb rump, preheat oven to 200°C
Boil kumara/potatoes in a pot of well-salted water for 8-10 minutes or until just tender.
Pat lamb dry with paper towels, season with salt and coat with spices. Heat oil in a large fry-pan on medium-high heat. Sear lamb for 2-3 minutes on each side until cooked medium-rare (or to your liking). If using lamb rump, you'll need to finish cooking the lamb in the oven for 5-10 minutes as it is thicker. Once cooked, leave to rest for 5-10 minutes before slicing against the grain. Wipe out fry-pan.
For the masala sweet potatoes, heat oil in fry-pan on medium heat. Cook onion, with a good pinch of salt, for a few minutes until soft. Add mustard seeds and curry powder and continue cooking for a further minute until fragrant. Turn off heat.
When kumara/potatoes are almost done, but have a couple more minutes of cooking to go, add peas and green beans to the boiling water to lightly cook. Drain all the veggies very well, then tip into spiced onion mixture, along with spinach/silverbeet, and toss everything together until greens have wilted. Season to taste with salt, pepper and a squeeze of lemon juice.
Mix yoghurt, mint and lemon juice together in a bowl and season with salt.
To serve, divide kumara/potatoes, vegetables and lamb between plates and drizzle with mint yoghurt.