Bring a large pot of lightly salted water to a boil. Add macaroni pasta and cook for 8 to 10 minutes or until al dente; drain.
In a 3 quart saucepan over medium heat, cook onion until transparent, about 10 minutes. Add butter and cook until melted. Stir in flour. Cook for about 1 minute, until smooth and bubbly; stirring occasionally. Mix in milk, cream cheese, salt, pepper, and Dijon mustard. Continue cooking until sauce is thickened. Add cooked macaroni, cooked bacon, cooked prawns and Cheddar cheese.
Pour into 2 quart casserole dish. In small bowl mix together bread crumbs, butter and parsley; spread over macaroni and cheese. Bake for 15 to 20 minutes or until golden brown and heated through.
Defrost. Bake uncovered at 400 degrees for about 30 minutes or until golden brown and heated through.