Melt butter in a large pan over a medium heat. Add onion and saute for 5 minutes or until well softened. Add cornflour, stir thoroughly, and cook for 30 seconds. Pour in wine and simmer for 3 minutes or until liquid is reduced by half.
Add cream and cheeses, reduce heat to low, and simmer for 5 minutes or until thickened. Season with salt and pepper, then stir.
Cook pasta in a large pan of rapidly boiling salted water for 2 minutes or until al dente. Drain well, then stir into sauce. Sprinkle with parsley and serve with bread.