Cook pasta shells according to package directions to al dente; drain and rinse in cold water. In a large skillet, cook beef and onion over medium heat until meat is no longer pink; drain. In a large bowl, combine the ricotta cheese, 1-1/2 cups Italian cheese blend, Parmesan cheese, pesto, egg and half of the beef mixture., Spread 3/4 cup spaghetti sauce into a greased 13x9-in. baking dish. Spoon cheese mixture into pasta shells; place in baking dish. Combine remaining beef mixture and spaghetti sauce; pour over shells. Sprinkle with remaining cheese. Cover and refrigerate overnight., Remove from the refrigerator 30 minutes before baking. Preheat oven to 350°. Cover and bake for 40 minutes. Uncover; bake until cheese is melted, 5-10 minutes longer.