In a heavy kettle, cook the onions with the prosciutto, garlic and chilies in 6 tablespoons of the oil over moderately low heat. Stir until they are softened. Add the cabbage and saute it over moderately high heat, stirring, for 3 minutes. Add the sugar, the tomatoes with the reserved juice, 3 tablespoons of the vinegar, the broth and the oregano. Bring the mixture to a boil, simmer it, covered for 15 minutes.
Let the mixture cool to warm, uncovered, and stir in the remaining 2 tablespoons olive oil, and the remaining 2 tablespoons vinegar. Season to taste. Transfer the cabbage mixture to a serving dish and sprinkle it with the parsley.