Heat oven to 350F.
In a bowl, combine the marinara and crab. Set aside.
Bring a large pot of water to a boil. Add a handful of kosher salt and the pasta shells. Cook to al dente as package directions dictate. Drain and rinse quickly with cold water.
While pasta is cooking, heat a large, nonstick skillet on medium-high heat. Add the oil, onions, and garlic. Saute until the onions are translucent. Add the spinach and cook, tossing, until spinach is nice and wilty. Transfer the spinach mixture to a cutting board and chop finely.
In a medium bowl, combine the chopped spinach mixture, ricotta, egg, salt, parsley, basil, pepper, parmesan, and 1 c mozzarella.
In a 9x13 baking dish, spread the crab marinara. One by one, take a pasta shell, spoon in a couple tablespoons of ricotta-spinach mixture, and place on the crab marinara in the baking dish. Cover baking dish with foil and bake at 350F for 30 minutes. Remove foil, top with remaining 1 cup mozzarella cheese, and bake another 5 minutes. Serves 6.