In a large pot of boiling, salted water, cook shells for about 9 minutes, until al dente. Drain and set aside
In a large skillet over medium heat, add Italian sausage and cook until just cooked through, about 5 minutes. Remove sausage and set aside. Wipe about 1/2 the oil out of the skillet
Add onion to skillet and cook, stirring occasionally, for 2 minutes, Add garlic, and cook for 1 minute. Add spinach and cook until wilted, about 3 minutes. Remove spinach mixture and set aside. Cool for 3-5 minutes, and chop well.
In a large bowl, add ricotta, 1/2 cup Parmesan, Mozzarella, artichoke hearts, spinach mixture, oregano, black pepper, pepper flakes, garlic powder, and eggs. Mix well to combine
Pour Marinara Sauce into the bottom of a baking dish. Fill shells with ricotta mixture, and place face-up on top of marinara sauce. Top with remaining Parmesan, and finish with crumbled Italian sausage.
Cover with aluminum foil, and bake for 30 minutes. Remove foil and bake, uncovered, for 5 minutes. Let rest for about 5 minutes before serving
I fit 10 shells into a 7x9 baking dish, so if you are making a full batch, use a 4.5qt baking dish, or 2 smaller dishes