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.
While pasta is cooking, heat butter and oil in a large skillet over low heat. Cook onion in this mixture until golden brown, 20 to 30 minutes. Remove from pan with a spoon and drain on paper towels.
In a large bowl, combine blue cheese, mascarpone and onion and mix well. Toss cheese mixture with spinach and pasta, season with salt and pepper and serve.