Make the herbed butter: mash the softened butter with the salt, pepper, garlic powder, onion powder, thyme and paprika.
Scatter the onion slices and celery in the bottom of a 6-quart slow cooker and pour in the broth.
Rub the turkey breast all over with the herbed butter, including a little under the skin where it lifts easily. Set the breast on the vegetables, skin side up.
Cover and cook on LOW for 5–6 hours, until a thermometer in the thickest part reads 165°F / 74°C. Avoid lifting the lid during cooking — each lift drops the heat.
Lift the breast onto a board, tent loosely with foil and rest 20 minutes. (For crisper skin, place it under a hot broiler for 3–5 minutes before resting — optional.)
Make the gravy: strain the cooking juices. Melt the 2 tbsp butter in a saucepan over medium heat, whisk in the flour and cook 1 minute. Slowly whisk in the juices and simmer 3–4 minutes until it coats the back of a spoon. Season to taste.
Slice the breast against the grain into thick slices, fan on a warm platter and serve with the gravy alongside.