Cook pasta in a large pot of lightly-salted, boiling water for 8 to 10 minutes or until al dente; drain. While pasta cooks, heat oil in a large skillet over medium high heat. Sauté onions until golden brown and then stir in garlic, and cook for two minutes longer. Remove from heat, and stir in balsamic vinegar. In a large bowl, combine pasta, onion mixture, and bleu cheese. Toss until evenly coated, and cheese is melted. Serve immediately.