In a large pot of salted boiling water, cook the pasta to al dente. Keep in mind that these need to be super undercooked, they will cook more in the slow cooker. They just need to be pliable enough to fill them, you don't want them to fall apart. Drain and set aside.
Meanwhile in a large bowl combine your ricotta cheese, spinach, 1 ½ cups mozzarella cheese and egg. Stir to combine well.
Spread just a bit of the sauce into the bottom of the slow cooker.
Spoon the ricotta mixture into the cooked shells.
Place the stuffed shells into the slow cooker. You can stack them to fit them all.
Add the browned, drained sausage all over the shells.
Next spread the remaining sauce all over the top.
Cook on low for 3-4 hours until bubbly.
During the last 30 minutes of cooking, top with the remaining shredded cheese.
Sprinkle with parmesan and parsley and serve!