Preheat the oven to 350 degrees.
Cook the jumbo shells as directed on the package. I like to add 2-3 tbsp of Kosher salt to the water when I cook pasta.
Cook the Italian sausage, let it drain on a plate or paper towel, and set it to the side. Taste the sausage once it is no longer pink. If the sausage is too mild & you like a little spicy, add a pinch of red pepper flakes to the sausage. Taste for salt & pepper as well.
While the pasta is cooking, mix together the ricotta, eggs, 2 cups of shredded mozzarella cheese, parmesan, Italian seasoning, and garlic powder. It will feel like you are adding too much garlic but the amount is perfect for the shells.
In the same pan you used to cook the sausage, quickly wilt the spinach. Use as much as you'd like. Over medium heat, add a little olive oil to the pan and then add the spinach. Sprinkle with salt & pepper. You don't have to cook it as down all the way - just enough to wilt it slightly. Once cooked, add the spinach to the bowl with the ricotta mixture.
Add the cooked sausage to the spinach and ricotta mixture and mix well.
Coat the bottom of a casserole dish with your favorite marinara sauce. When the shells have finished cooking, spoon the mixture into shells one by one and put them on top of the marinara sauce. When you've stuffed all the shells, spoon marinara sauce on top of each shell. Top with shredded mozzarella cheese.
Cover the dish and cook for 30 minutes. If the cheese is not fully melted, uncover the dish and cook for an additional 10 minutes.