Heat the oil and butter in a large skillet over medium heat. Add the sliced onions and cook, stirring occasionally, until softened and lightly golden. Transfer the onions to a plate and set aside.
Season the chicken breasts evenly with garlic powder, paprika, and black pepper.
Using the same skillet, sear the chicken breasts just until lightly browned on both sides. The chicken should not be fully cooked. Remove and set aside.
In a large bowl, stir together the cream of chicken soup, chicken broth, remaining garlic powder, black pepper, uncooked white rice, and the dry onion soup mix until well combined.
Spread the rice mixture evenly in the slow cooker. Arrange the seared chicken on top, then spoon the caramelized onions over the chicken.
Cover and cook on High for 3 hours or on Low for 5 hours, until the chicken is cooked through and the rice is tender.
Lay the Swiss cheese slices over the chicken, cover, and let sit for about 5 minutes until melted. Garnish with fresh parsley and serve warm.
