1. Make the Polenta: Preheat the oven to 400°. Bake the sweet potato for about 20 minutes, or until barely tender. Peel and cut the sweet potato into 1/2-inch dice. Raise the oven temperature to 500°.
2. Bring the chicken stock to a boil in a medium saucepan. Lightly oil a shallow 8-inch bowl. Whisk the polenta and 1/2 teaspoon of salt into the stock and cook over low heat, stirring often, until the polenta thickens and begins to pull away from the side of the pan, about 12 minutes. Stir in the Parmesan cheese. Remove from the heat and season the polenta with salt and pepper, then gently fold in the diced sweet potato. Spoon the polenta into the prepared bowl and smooth the surface. Cover with plastic wrap and refrigerate until firm.
3. Heat 1 teaspoon of the olive oil in a large cast-iron skillet. Cut the polenta into 8 wedges and cook over moderate heat until browned on the bottom, about 3 minutes. Turn the polenta with a metal spatula, add the remaining 1 teaspoon of oil to the skillet and tilt to coat the bottom. Bake the polenta for about 8 minutes, or until crusty on the bottom.
4. Make the Salad: In a large bowl, whisk the orange juice with the sherry vinegar and garlic. Season with salt and pepper. Add the parsley, ham, apple and onion to the bowl and toss well. Arrange 2 wedges of warm polenta on each of 4 plates, mound the ham salad in the center and serve.