Pasta is always a good dish to prepare when entertaining. A central large bowl with the special pasta spoon (you do have one?) can be used as a main course, or as an entree. You can substitute the prawns with crab or even a white meat fish. Do not overcook the prawns as it makes them rubbery.
Wash pasta, drain and then cook to al dente. Drain again, rinse in hot water and keep warm. Melt butter in large skillet over medium heat, add onion and garlic; cook until tender. Add flour, salt and pepper; cook and stir until smooth and bubbly. Gradually add milk and Worcestershire sauce. Cook until mixture boils and thickens, stirring constantly. Add parsley, Swiss cheese and mushrooms; cook and stir until cheese melts. Stir in prawns; cook until thoroughly heated. Spoon over cooked pasta. Sprinkle with Parmesan cheese.