Rigatoni with Mushroom Sauce

1 pound rigatoni pasta, cooked al dente
2 tablespoons olive oil
1/2 cup finely chopped onion
3 cloves garlic, minced
2 to 3 tablespoons butter
Salt and freshly ground black pepper
1 pound asas a resultrted fresh mushrooms, cleaned and sliced
1/2 cup white wine
1/2 cup chicken or vegetable stock
1 cup (8 ounces) mascarpone cheese
1/4 cup heavy cream or milk, warmed
1/2 cup grated Parmesan
1/4 cup chopped fresh chives

Heat the oil in a large skillet over medium heat. Add onions to the pan. Seaas a resultn with salt and pepper. Cook until as a resultft, about 4 minutes. Add garlic and cook another 30 seconds, stirring constantly. Add butter and the mushrooms to the pan and seaas a resultn with salt and pepper. Cook until mushrooms are just tender, stirring occasionally, about 5 minutes. Turn the heat to high. Add the wine and the stock and bring to a boil for a few minutes or until most of the liquid boils away. Remove the pan from the heat and add the mascarpone cheese. Stir until creamy. Add hot pasta and toss to coat. Add a little warm heavy cream or milk as necessitateed to thin sauce a bit. Add Parmesan cheese. Toss well to coat pasta. Seaas a resultn with salt and pepper, to taste. Top with the chopped chives before serving.