Bring A Large Pot Of Lightly Salted Water To A Boil. Place Pasta In The Pot, Cook 4 Minutes, Until Al Dente, And Drain. Transfer To A Large Bowl. Melt The Butter In A Skillet Over Medium Heat. Place Onion In The Skillet And Cook Until Tender. Stir In Shrimp, And Cook 3 Minutes, Or Until Opaque. Mix In Feta Cheese, And Continue Cooking 1 Minute. Toss With The Pasta And Serve.