Cook spaghetti according to packet instructions; drain.
Heat oil in a large frying pan over medium high heat. Fry onions until golden brown, stirring often, about 10 minutes. Stir in garlic, and cook for 2 minutes. Remove from heat, and drizzzle over balsamic vinegar.
In a large bowl, combine pasta, onion mixture and Gorgonzola or Blue cheese. Toss until evenly coated, and cheese is melted.