Prepare the Slow Cooker: Lightly coat the ceramic portion of a 6 to 7-quart slow cooker with cooking spray. Scatter the onion chunks over the base of the cooker.
Season the Turkey: Pat the turkey breast dry. In a small bowl, combine softened butter, garlic, thyme, rosemary, sage, salt, paprika, and pepper. Rub this butter mixture over the turkey breast, including underneath the skin if possible, taking care not to tear it.
Cook the Turkey: Place the seasoned turkey breast on top of the onions in the slow cooker. Pour the orange juice and reduced sodium chicken broth around the turkey. Cover with the lid and cook on HIGH for about 3 hours or on LOW for about 5 hours, until the internal temperature reaches 160°F. Use a digital thermometer for accuracy.
Optional Crispy Skin: For crispy skin, preheat the broiler to 500°F. Transfer the cooked turkey breast to a baking sheet, place on a wire rack, and broil for 5 minutes or until the skin is golden and crispy. Keep a close watch to prevent burning.
Make the Gravy (Optional): Strain the cooking juices from the slow cooker, discarding onion chunks. Add additional broth if necessary to make 2 cups. In a saucepan, melt the butter over medium-high heat, then whisk in the flour to make a roux. Gradually add the broth, whisking constantly, and simmer for 5 minutes or until thickened. Season with salt and pepper to taste.
Serve: Slice the turkey breast as desired and serve with gravy.