In a small bowl, combine the softened butter, minced garlic, chopped rosemary, thyme, sage, lemon zest, kosher salt, black pepper, and smoked paprika. Mix thoroughly until a uniform compound butter forms. Set aside.

Place the onion wedges, smashed garlic cloves, and fresh herb sprigs in the bottom of a 6-quart slow cooker. Pour in the chicken broth. This aromatic base will flavor your drippings and keep the bottom of the turkey from sitting directly on the heating element.

Pat the turkey breast completely dry with paper towels — this is critical for good skin. Using your fingers, carefully loosen the skin from the breast meat without tearing it. Push about half of the herb butter directly under the skin and spread it evenly over both lobes of the breast.

Rub the remaining herb butter all over the exterior of the turkey breast, including the sides and underneath. Season the whole surface with a light additional sprinkle of kosher salt.

Place the herb-buttered turkey breast skin-side up in the prepared slow cooker on top of the onion and herb base. Place the lid on securely.

Cook on LOW for 5 to 6 hours, or until an instant-read thermometer inserted into the thickest part of the breast (away from the bone) reads 165°F. Do not cook on HIGH if possible — LOW heat produces significantly juicier, more evenly cooked meat. Begin checking temperature at the 5-hour mark.

Preheat your oven broiler to HIGH. Line a rimmed baking sheet with foil. Carefully transfer the cooked turkey breast to the prepared baking sheet, skin side up.

Broil 6 to 8 inches from the heating element for 3 to 5 minutes, watching constantly, until the skin is deep golden brown and crisped. Do not walk away during this step — the skin can go from golden to burnt in under a minute. Remove and let the turkey rest for 10 minutes before carving.

Pour all of the drippings from the slow cooker through a fine mesh strainer into a medium saucepan. Skim off as much surface fat as you like using a spoon or fat separator.

Bring the drippings to a simmer over medium heat. In a small bowl, whisk together the flour (or cornstarch) and cold water until smooth. Slowly whisk the slurry into the simmering drippings. Add soy sauce if using. Cook, whisking constantly, for 3 to 5 minutes until the gravy has thickened to your desired consistency. Taste and season with salt and pepper.

Carve the turkey breast by running a sharp carving knife along the breastbone, then slicing the meat crosswise into even portions. Arrange on a platter, drizzle generously with gravy, and garnish with fresh rosemary or thyme sprigs. Serve immediately.