Thinly slice the chicken breasts at a slight angle. Toss in a medium bowl with the cornstarch, salt, and pepper to taste.
In a large wok or pan, drizzle oil and heat over medium high heat. Once thoroughly heated, fry the chicken until cooked through, stirring occasionally. Use a slotted spoon to remove the chicken to a towel lined plate.
Add the garlic, ginger, and onion to the wok. Cook, stirring occasionally, until softened, but not brown. Stir in the pineapple pieces, soy sauce, and chicken. Cook until everything is heated through. Add the white parts of the spring onions and half of the sliced green parts.
Serve hot with rice and remaining sliced green onions as garnish.