1. Insert the sprigs of rosemary under the skin of the turkey, then lay it skin side down and season with salt and pepper. Tie the breast with butcher's twine to form a compact cylinder and season the skin with salt and pepper.
2. Melt 2 tablespoons of the butter in a large skillet over medium heat. Add the turkey and cook until light brown on all sides, 8 to 10 minutes. Transfer the turkey to a plate and set aside.
3. Reduce the heat to medium-low, add the onions, nutmeg, and salt as desired, and cook stirring occasionally, until the onions dry out and darken, 30 to 35 minutes.
4. Place the turkey on top of the onions and pour in the wine and broth. Cover and cook until the breast is tender, about 30 minutes. Transfer the breast to a work surface.
5. Increase the heat to medium and cook until the cooking liquid reduces and thickens, about 5 minutes. Turn off the heat and swirl in the remaining 2 tablespoons butter.
6. Untie the breast and slice into serving pieces. Pour any collected juices into the onions. Scrape the onions onto a platter, arrange the sliced turkey on top, and serve.