Bring a large pot of salted water to a boil and add the jumbo pasta shells. You'll need about 16 shells, so cook 20+ in case some break apart while boiling. Boil the shells according to the package instructions (9-12 minutes usually), then drain. Put the shells on a baking sheet to cool, and drizzle with half the oil to keep them from drying out or sticking to each other.
Heat a skillet over medium heat. Add the remaining olive oil and let it heat up for 1-2 minutes. Add the garlic and saute for 30 seconds until fragrant. Then, add the spinach, salt, and pepper and cook until the spinach is wilted, 3-5 minutes.
Set aside to cool. Once the spinach is cooled, squeeze the liquid out using either your hands, a paper towel, or a cheese cloth. You don't have to get all of the liquid out, just most of it (this keeps your filling from being runny). Once the liquid is out, roughy chop the spinach and set aside.
In a large mixing bowl, mix together the ricotta, half the mozzarella, Parmesan, egg, sausage, parsley, basil, spinach and salt/pepper. Mix until everything is fully incorporated.
Prepare the baking dish. Pour all the marinara in the bottom of a 9x13 dish. Preheat the oven to 350 F (175 C).
Stuff the shells. You can do this with a spoon, or you can put the filling into a gallon-sized bag and cut a large piece off the corner (this is a chunky mixture!). Add 2-3 TBSP of filling to each shell. Place the shells in the marinara as you complete them. Once they are all filled, top with the remaining mozzarella.
Bake covered for 35 minutes. If desired, broil for an additional 2-3 minutes to brown the cheese (keep a close eye on it!). Let cool slightly before serving. Let cool fully before storing.
