1.Using sharp knife, score lamb at 2cm intervals. Combine lamb, oil, garlic, mustard, rind and juice in large bowl. Cover; refrigerate 3 hours or overnight.
2.Preheat oven to 200°C (180°C fan-forced).
3.Place lamb on oiled wire rack over large baking dish. Roast, uncovered, about 45 minutes. Cover lamb; stand 10 minutes, then slice thinly.
4.Meanwhile, separately boil, steam or microwave potato and beans until tender. Drain. Push potato through fine sieve into large bowl, then stir in butter and milk until smooth. Place beans in a small bowl and crush coarsely with fork. Fold beans into potato mixture.
5.To make mint sauce, blend or process mint and garlic until smooth. With motor operating, gradually add oil in a thin steady stream, blending until mixture is smooth. Stir in vinegar and sugar.
6.Serve lamb with mint sauce and broad bean mash.