1. Preheat the oven to 400°. Put the turkey breast in a very large roasting pan. Brush the melted butter all over the turkey breast and season it with salt and pepper. Add the lemon halves to the pan, cut side down. Roast the turkey for about 45 minutes, or until the lemon halves are browned on the cut side. Transfer the lemon halves to a plate and baste the turkey.
2. Add the olive oil to the pan. Push the turkey to 1 side and add the potatoes. Turn the potatoes in the fat until coated all over and arrange them, cut side down, around the turkey. Season with salt and pepper.
3. Roast the turkey for 1 hour longer, basting the turkey and potatoes every 20 minutes. The breast is done when an instant-read thermometer inserted in the thickest part (but not touching the bone) registers 160°.
4. Increase the oven temperature to 500°. Transfer the turkey to a carving board and cover loosely with foil. Transfer the potatoes to a bowl. Pour the pan juices into a large glass measuring cup and skim off the fat. Add 1/4 cup of the fat to the roasting pan; reserve the remaining fat. Return the potatoes to the roasting pan, cut side down, and roast in the bottom third of the oven for about 30 minutes longer, or until deeply browned. Using a metal spatula, transfer the potatoes to a warmed bowl.
5. Meanwhile, in a medium saucepan, heat 2 teaspoons of the reserved fat. Add the lemon halves, cut side down, and the soy sauce and cook over moderately high heat for 1 minute. Stir in 3 cups of the stock and bring to a boil, then simmer over moderately low heat for 5 minutes. Remove the lemon halves, and when cool enough to handle, squeeze the juice into the saucepan. Add the reserved pan juices.
6. In a small bowl, whisk the remaining 1/2 cup of stock with the cornstarch until smooth. Whisk the slurry into the saucepan and bring to a boil over moderately high heat. Simmer until the jus is slightly thickened, about 1 minute. Season the jus with salt and pepper and transfer to a warmed gravy boat.
7. Carve the turkey breast and serve with the roasted potatoes, passing the lemon-soy jus.
Wine Recommendation: A light, fruity Beaujolais will blend with the mild flavors here. Try the 2002 Château de la Chaize Brouilly or a grapey, just-arrived 2003 Beaujolais Nouveau.