This is really good and easy to make. A good family casserole. I can't remember where I got this recipe but it called for plain spaghetti sauce and I like lots of flavor so I changed it up to suit my tastes.
Cook pasta shells according to pkg directions to al dente. drain and rinse in cold water. In a lg skillet, cook beef and onion till onion is tender. and meat is done. drain. In a bowl, combine the ricotta cheese, 1 1/2 cups of the Italian cheese blend, parmesan cheese, pesto, egg, and half the cooked beef. In a greased 13x9 inch pan, Spread 3/4 cup the spaghetti sauce on bottom. Spoon the cheese mixture in the cooked pasta shells. Place shells in baking dish. Combine remaining spaghetti sauce with remaining ground beef. Spoon over shells. Sprinkle with remaining cheese.
Cover and bake at 350 degrees F for 30 minutes. Uncover and bake 10 minutes longer.