Place 1 pound boneless skinless chicken breasts (cut into 1-inch cubes) and 1/4 cup cornstarch in a large zip-top bag. Seal and shake until the chicken is evenly coated.
In the slow cooker, whisk together 2 tablespoons olive oil, 1 teaspoon minced garlic, 1/2 cup low-sodium soy sauce, 1/2 cup water, 1/2 cup brown sugar, and 1 cup grated carrots until combined.
Add the coated chicken to the slow cooker and stir to coat the chicken evenly with the sauce.
Cover and cook on LOW for 4–5 hours or HIGH for 2–3 hours, until the chicken is fully cooked (internal temperature 165°F/74°C) and tender.
Stir once more to combine, then garnish with green onions and serve.