Mix 2 tablespoons cornstarch, garlic powder, and ground ginger together in a small bowl. Toss chicken thighs in cornstarch mixture until they are evenly coated.
Heat oil in a skillet over medium-high heat. Cook chicken in the skillet until browned on all sides, 3 minutes per side. Transfer to a plate and set aside.
Drain juice from the can of pineapple chunks into a measuring cup. You should have approximately 3/4 cup to 1 cup of juice.
Mix 1/2 cup pineapple juice, chicken stock, rice vinegar, brown sugar, ketchup, and soy sauce together in a slow cooker. Add browned chicken pieces and stir to coat chicken with the sauce. Cook on Low for 3 1/2 hours.
Whisk remaining 2 tablespoons cornstarch with remaining 1/4 cup pineapple juice. Stir cornstarch mixture into the slow cooker. Increase the slow cooker heat to High and add pineapple chunks and chopped red bell pepper.
Cook on High until the sauce is bubbly and thickened and bell pepper is crisp-tender, about 30 minutes. Garnish with chopped green onion.
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