Remove the turkey breast from the refrigerator at least a half hour before cooking to take the chill off. Preheat oven to 350 degrees. Rub the meat with 2 Tbsp. olive oil, then salt and pepper generously. Place the meat in a baking dish and add just enough stock and wine to cover the bottom third of the meat. (You may need more or less, depending on the size of the breast and dish.) Squeeze 1 Tbsp. juice from the lemon and reserve for later. Add the lemon halves and garlic to the broth-wine mixture. Transfer to the oven, uncovered, and cook for about 1 hour, or until an instant-read thermometer reads 150. Transfer the turkey to a plate to cool, reserving 1⁄4 cup of the pan juices.
Make the sauce: In a blender combine the tuna (and the packing oil) with 1⁄2 cup olive oil, 2 Tbsp. wine, reserved lemon juice, anchovy paste, capers, and reserved pan juices. Purée until smooth, then season with pepper. (Taste for salt, but the anchovies should supply enough.)
To serve, slice the turkey across the grain and dress with the sauce. Garnish with the radish slices, watercress, and cherry tomatoes.