Chop 6 onions. In a large skillet, melt 1 tablespoon unsalted butter in 2 tablespoons olive oil. Add the onions, 1 teaspoon salt and 1 teaspoon sugar and cook over moderate heat, stirring frequently, until the onions are well browned, 20 to 30 minutes. Transfer the onions to a bowl.
Meanwhile, stem 2 pounds of Portobello mushrooms. Halve the caps and slice them crosswise 1/4 inch thick. In the same skillet, melt 1 tablespoon unsalted butter in 1 tablespoon olive oil. Add the mushrooms and 1/2 teaspoon salt and cook over moderate heat, stirring occasionally, until tender and browned, about 8 minutes. Add the reserved onions and 6 tablespoons chopped fresh flat-leaf parsley. Season with salt and pepper.
In a large pot of boiling salted water, cook 1 1/2 pounds of ziti until al dente, about 13 minutes; reserve 1 1/2 cups of the pasta water and drain the ziti.
In a large bowl, toss the ziti with 1 cup of the reserved pasta water, the mushroom mixture, 8 ounces crumbled soft goat cheese, 3 tablespoons freshly grated Parmesan and 1 tablespoon olive oil. If the pasta seems dry, add more of the pasta water. Serve immediately, passing additional grated Parmesan at the table.
This dish needs a red with plenty of acidity, try a Santa Cristina.