Heat the oven to 375 degrees. Grease a 9-by-13-inch baking/casserole dish with olive oil or oil spray.
Cook the pasta shells in boiling salted water for 1 minute less than 'al dente' on the package instructions. Drain in a colander then set aside until cool enough to handle.
Meanwhile, in a large bowl mix the cottage cheese/ricotta, spinach, parmesan, 1 cup shredded mozzarella, salt, garlic powder, pepper, and pesto (if using).
Spread 1 cup of marinara sauce in the bottom of the baking dish.
Stuff each pasta shell generously with the cheese mixture. (I use a spoon but you can also use a piping bag or plastic bag with the corner snipped off to squeeze in the filling.) Place the shells lined up tightly next to each other on top of the marinara sauce in the baking dish. If you have any leftover cheese filling, just spoon a little on top of some of the shells.
Once all of the shells are filled and in the pan. Spoon the remaining 1 cup of marinara sauce over the shells. They don't have to be completely covered. Sprinkle the remaining 1 1/2 cups of shredded mozzarella over the top.
Bake until the cheese is bubbly and golden brown, 15 to 20 minutes. Allow to cool for 5 to 10 minutes then scoop the cheesy shells onto plates and serve.
Leftover shells can be stored in the fridge for 4 days.