In a small bowl, combine the softened butter, kosher salt, black pepper, garlic powder, oregano, thyme, and paprika. Mix with a fork until it forms a smooth paste.
Arrange the sliced onion in an even layer on the bottom of a 6-quart or larger slow cooker.
Pat the turkey breast completely dry with paper towels. If the breast has skin, gently loosen it from the meat with your fingers.
Spread about two-thirds of the herb butter mixture directly onto the meat, under the skin if present. Spread the remaining butter all over the outside of the turkey.
Place the seasoned turkey breast on top of the onions in the slow cooker.
Cover and cook on LOW for 4 to 5 hours. The turkey is ready when a meat thermometer inserted into the thickest part registers 165°F (74°C).
For crispy skin (optional): Carefully transfer the cooked turkey to a baking sheet lined with foil. Place under the broiler for 2 to 5 minutes, watching constantly, until the skin is golden and crisp.
Transfer the turkey to a cutting board and let it rest for 15 minutes before slicing. This helps the juices settle, ensuring moist meat.