Pre-heat the oven to 350º F and cook the pasta shells as per the directions on the box. When done, drain the pasta shells, run under cool water to stop the cooking process, and drain again to make sure there's no remaining water inside of the shells. Set aside.
While the pasta is cooking, you can bring your stuffing together. Add the strained ricotta cheese to a large bowl. Drain the oil from the cans of mackerel, keeping the other ingredients like the olives and seasonings in the can. With a fork, flake the mackerel into the bowl with the ricotta cheese.
Add the spinach, olives, garlic powder, shredded cheese, beaten egg, and a pinch of salt and pepper. Finally, with a large spoon or spatula, mix all of these ingredients thoroughly until evenly distributed.
Into the baking dish, pour just enough marinara to coat the bottom of the dish so the shells don't stick. This should be about 1/2 cup, depending on the size of your baking dish.
With a tablespoon, carefully stuff each pasta shell with the mixture, starting with a heaping tablespoon's worth of stuffing for each shell, then place the shells into the baking dish. In the end, if you have some mixture left over, go ahead and overstuff a few shells until it's gone.
Spoon 1-2 cups of marinara over the shells. The amount of sauce really depends on how saucy you want them. It's a personal preference, so you do you :) But if you aren't going to cover the dish with tinfoil before baking, I'd definitely add a little extra sauce for the sake of keeping moisture.
Sprinkle some extra shredded cheese over the shells.
Finally, cover the shells with tinfoil if you have it, and bake in the over for 20-25 minutes.
When done, remove from oven, carefully remove the tinfoil, and sprinkle the shells with freshly chopped parsley before serving.