Melt the butter in an enameled cast-iron casserole. Add the carrots and onion and cook over moderately high heat, stirring, until the onion is lightly browned, about 8 minutes. Add the cabbage, stock and sugar, season with salt and pepper and toss to coat. Cover and cook over low heat, stirring, until tender, about 15 minutes. Uncover, raise the heat to high and cook, stirring, until most of liquid has evaporated, about 5 minutes. Stir in the prosciutto, parsley and thyme, cook for 1 more minute and serve.