Place the diced chicken breast pieces into the basin of a 4 to 4½-quart crockpot.
In a separate bowl, thoroughly whisk together the pineapple juice, hoisin sauce, soy sauce, brown sugar, and minced garlic until well blended.
Reserve ¼ cup of the sauce mixture and pour the remainder over the chicken in the slow cooker. Gently stir to ensure the chicken is evenly coated.
Secure the lid on the crockpot and set it to cook on LOW for 4 hours.
After the 4-hour mark, remove the lid. In a small bowl, combine the reserved ¼ cup of sauce with the cornstarch, whisking until smooth and free of lumps.
Pour the cornstarch slurry into the crockpot and stir gently to incorporate.
Switch the crockpot to HIGH and continue to cook uncovered for an additional 30 to 45 minutes, or until the sauce has thickened to your desired consistency.
Before serving, garnish the dish with thinly sliced green onions.