Penne with Blue Cheese, Tomatoes, and Spinach

Bring a large pot of lightly salted water to a boil for the pasta. In a large, deep skillet, heat oil and butter until butter melts. Stir in leeks and cover; cook over low heat 10 minutes, stirring once or twice, until softened. Cook pasta in boiling water according to package directions. Stir crème fraîche, mustard, and pepper into leeks until blended, then stir in 3/4 cup of the blue cheese until melted. Remove from heat; stir in tomatoes. Drain pasta, reserving 1/3 cup of the cooking water. Stir pasta into sauce, adding cooking water so the sauce coats the pasta. Stir in spinach until it wilts. Transfer to a serving bowl. Top with remaining blue cheese.