Cut beef across grain into thin strips; mix with 1 tbsp (15 mL) of the soy sauce, cornstarch and 1/2 tsp (2 mL) pepper.
Cut carrots into thirds; thinly slice lengthwise.
Grate 1/4 tsp (1 mL) of the orange rind; place in small bowl.
Juice orange (you should have 1/2 cup/125 mL), discarding pulp; add juice to bowl with remaining soy sauce, rice vinegar and sesame oil; stir in generous pinch of sugar until dissolved. Set aside.
In wok or skillet, heat 1 cup (250 mL) oil over high heat; fry half of the beef until lightly browned, 30 to 60 seconds. Using slotted spoon, transfer to sieve set over bowl to drain any remaining oil. Repeat with remaining beef. Fry carrots until edges are browned, 4 to 5 minutes; using slotted spoon, transfer to sieve.
Pour out all but 1 tbsp (15 mL) of the oil. Add garlic and pinch of salt; stir-fry until golden, about 20 seconds. Add orange mixture; when boiling, add beef and carrots. Stir-fry until sauce is thickened, about 30 seconds. Transfer to serving plate and sprinkle with onion.