Heat oil in a wok or frying pan over medium-high heat. Cook chicken, in batches, stirring, until golden. Transfer to a bowl.
Add onion to wok or pan. Cook, stirring, for 5 minutes or until softened. Add garlic. Cook for 1 minute or until fragrant. Return chicken to pan.
Add soy sauce. Cook, covered, for 5 minutes, or until chicken is cooked through.
Add sugar. Cook, stirring, for 1 to 2 minutes or until mixture is bubbling. Add fish sauce and juice. Cook, stirring, for 1 minute or until sauce thickens.
Serve with pickled cucumber and rice.
Pickled Cucumber
Place vinegar, sugar in a saucepan over medium heat.
Cook, stirring, for 2 minutes or until sugar has dissolved. Transfer to a heatproof bowl.
Add cucumber. Stir to combine. Set aside to cool completely. Stir in onion.