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.