Preheat oven to 350 degrees. Spread 3/4 cup of the spaghetti sauce on bottom of 13x9-inch baking dish. Mix egg, cheeses, and pesto sauce until well blended. Spoon cheese mixture into large resealable plastic bag. Using scissors, cut off small hole from one of the bottom corners of bag.
Cook manicotti shells for half of the time on the box. This will keep them firm enough to handle and not fall apart when stuffing. The additional cook time in the oven will cook them the rest of the way. After you remove the partially cooked manicotti from the stove, transfer them into a container of cold water. This will prevent them from continuing to cook
Fill manicotti shells, 1 at a time, squeezing cheese mixture into both sides of each shell. Place manicotti over sauce in dish; pour remaining 1 1/4 cups spaghetti sauce over manicotti. Cover with foil.