1.Combine garlic, cornflour, half the sauce and half the sherry in a large bowl; add lamb, mix well.
2.Heat peanut oil in a wok over high heat; stir-fry lamb mixture, in batches, until browned.
3.Return lamb to wok with sugar, sesame oil and remaining sauce and sherry; stir-fry until sauce thickens slightly. Remove from heat; serve stir-fry sprinkled with onion.
The lamb will be easier to cut if you freeze it for about 20 minutes before slicing.Note