Preheat the oven to 375°F (190°C) and lightly spray a 9×13-inch baking dish with cooking spray.
Cook the jumbo pasta shells according to package directions until al dente, then drain and set aside to cool slightly.
Pour 1/2 cup of the spaghetti sauce into the prepared baking dish and spread into an even layer.
In a mixing bowl, combine 1½ cups cottage cheese, 1½ cups shredded mozzarella, 1/4 cup grated parmesan, 1/3 cup pesto, and 1 large egg until well mixed.
Stuff each cooked shell with the cheese–pesto mixture and place them seam-side up on top of the sauce in the baking dish.
Spoon the remaining spaghetti sauce over the stuffed shells, then sprinkle the remaining 1/2 cup shredded mozzarella evenly over the top.
Bake uncovered in the preheated oven for 30 to 35 minutes, until the sauce is bubbling and the cheese is melted and lightly golden.
Let the casserole rest a few minutes before serving.