1. Partially freeze chicken for easier slicing, then slice thinly or cut into cubes.
2. Coat chicken with 1/4 cup cornstarch, shaking in a resealable bag or mixing well.
3. Heat oil in a pan or wok over medium-high heat and fry the chicken in batches until golden and crispy, 3-5 minutes per batch. Drain on paper towels.
4. Remove excess oil, then add garlic, ginger, and white parts of onions to the pan; cook for 15-30 seconds until fragrant.
5. Add soy sauce, brown sugar, sesame oil, and stock to the pan; bring to a simmer.
6. Mix remaining cornstarch with water to create a slurry, stir into the sauce until thickened.
7. Return chicken and green onion tops to the pan; toss to coat and heat through.
8. Serve hot over rice or noodles.