Before you start, preheat oven to 220°C (or 200°C fan bake).
Prep potatoes and onion and toss with yams and oil on a lined oven tray. Season with salt and pepper and roast for about 35 minutes, until golden and cooked through.
Prep broccoli, mint and second measure of onion and set aside.
Pat lamb dry and season with salt and pepper. Heat oil in a large fry-pan on medium-high heat. Cook lamb for 3 minutes each side, until browned (but not cooked through). Transfer lamb to a lined oven tray and cook for 8-14 minutes for medium-rare (depending on thickness), or until cooked to your liking. Rest, covered. Reserve pan for gravy.
Bring a medium pot of salted water to the boil. Add broccoli and peas to pot and cook for about 5 minutes, until bright green and tender. Drain well and return to pot along with mint and butter. Mash and season to taste with salt and pepper. Cover to keep warm.
Return reserved pan to medium heat. Cook onion for about 4 minutes, until soft. Add gravy spices and cook for 1 minute, stirring. Slowly add water measure to pan, stirring to prevent lumps forming. Cook for a further 3-4 minutes, until thickened. Season to taste with salt and pepper.
Thinly slice lamb against the grain and add any resting juices to gravy.
Serve lamb, veggies and broccoli smash topped with gravy.