Pat the turkey breast completely dry with paper towels. Drying the surface is the single most important step for getting good flavor and a non-rubbery skin, so do not skip it.

In a small bowl, mix together the softened butter, minced garlic, thyme, rosemary, sage, 1 teaspoon of salt, black pepper, and onion powder until everything is evenly combined and the herbs are distributed throughout the butter.

Using your fingers, gently loosen the skin away from the breast meat, working carefully so the skin stays attached at the edges. Slide about half of the herb butter directly under the skin and press from the outside to spread it across the meat. Rub the remaining butter all over the outside of the turkey, coating the skin and any exposed meat evenly. Season the outside with a light additional pinch of salt.

Pour the broth into the bottom of the slow cooker. Place the turkey breast in the slow cooker skin-side up. The broth keeps moisture in the bottom of the pot and prevents scorching while the turkey cooks.

Cover and cook on LOW for 6 to 7 hours. Do not cook on HIGH, as that makes the meat tighter and drier. Start checking at the 6-hour mark by inserting an instant-read thermometer into the thickest part of the breast without touching bone. You are looking for 165 degrees F. If it needs more time, check again every 30 minutes.

Once the turkey hits 165 degrees, carefully transfer it to a cutting board and tent loosely with foil. Let it rest for 10 minutes before slicing. Resting lets the juices redistribute so they stay in the meat instead of running out when you cut it.

Optional pan sauce: Pour the cooking juices from the slow cooker into a small saucepan over medium heat. In a small cup, whisk together the cornstarch and cold water until smooth, then stir it into the juices. Cook, stirring constantly, for 2 to 3 minutes until the sauce thickens slightly. Taste and adjust salt. Spoon over sliced turkey when serving.