Combine curry powder, garlic powder, and pepper in a small bowl. Sprinkle the mixture evenly over the chicken thighs, pressing gently to help the spices adhere.
Place the seasoned chicken in a 3-quart slow cooker. Add the sliced onions on top.
Carefully pour the marinade along the sides of the slow cooker, avoiding pouring directly over the chicken to preserve the spice coating.
Cover and cook on low for 4-6 hours, or until the chicken reaches an internal temperature of 1706F.
Transfer the chicken to a plate and keep warm. Reserve the cooking juices.
In a large saucepan, heat the canola oil over medium heat. Whisk in the flour and cook for 1-2 minutes until smooth.
Gradually whisk in the reserved cooking juices. Bring the mixture to a boil, stirring constantly, and cook until thickened, about 1-2 minutes.
Reduce the heat to low, return the chicken to the saucepan, and simmer for 5 minutes to let the flavors meld.
Serve hot with cooked rice, garnished with chopped green onions and cilantro, if desired.