Heat oil in a hot wok until surface seems to shimmer slightly. Add half of the beef and stir-fry for 2 minutes. Remove beef from wok with a slotted spoon and drain on kitchen paper. Add extra oil to wok with remaining beef and stir-fry for 2 minutes.
Return all beef to wok, along with garlic and onion, and stir-fry for 2 minutes.
Add sugar and soy sauce and stir-fry for 3 minutes, or until beef is tender and just cooked. Serve immediately, sprinkled with Sichuan pepper and salt, and with a side dish of fresh lemon juice.
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