Bring a large pot of lightly salted water to a boil. Add pasta and cook for 8 to 10 minutes or until al dente; drain.
Meanwhile, melt butter together with the olive oil in a saucepan over medium heat. Stir in onion and cook until softened and translucent, about 2 minutes. Stir in garlic, capsicum and mushroom; cook over medium-high heat until soft, about 2 minutes more.
Stir in the prawns and cook until firm and pink, then pour in Alfredo sauce, Romano cheese and cream; bring to a simmer stirring constantly until thickened, about 5 minutes. Season with cayenne, salt and pepper to taste. Stir drained pasta into the sauce and serve sprinkled with chopped parsley.