Preheat oven to 400 degrees. In a small saucepan, combine the rice, ½ teaspoon salt, and 2 cups water. Bring to a simmer, cover, and cook until rice is al dente, about 14 to 15 minutes. Spread on a baking sheet to cool.

Peel the onions and halve crosswise. Cut a small piece from each end so the onions will sit up straight, making a cup shape. Cut and scoop out the interior of the onion halves to leave shells that are about 2 or 3 layers thick. Chop the onion flesh that you scooped out, and set aside. In a large bowl, toss the onion shells with 2 tablespoons olive oil and ½ teaspoon salt.

Heat the remaining 4 tablespoons olive oil in a large skillet over medium heat. Add the chopped onion pieces, and cook, stirring often, until soft, about 10 minutes. Add the diced ham, and cook until the ham is just golden on the edges, about 3 minutes. Scrape into a large bowl, and let cool slightly, about 10 minutes.

To the large bowl, add the cooked rice, the roasted peppers, grated cheese, parsley, egg, and remaining ½ teaspoon salt. Toss well to combine and make a cohesive stuffing. Mound the filling into the onion shells, and set them in a baking dish large enough to hold the onions without crowding them.

Pour the stock and wine into the dish, and scatter 2 tablespoons of the butter in the liquid. Dot the tops of the onions with the remaining 2 tablespoons butter. Cover the pan with foil (without letting it touch the onions), and bake until the onions are tender, about 35 minutes. Uncover, and bake until the tops are brown and crusty, about 20 to 25 minutes more. Serve hot, with the pan juices, or at room temperature.