Put pasta shells in a large pot of boiling salted water. Cook until just slightly softened but still firm, about 7 minutes. Drain and rinse under cold water. Drizzle with olive oil and set aside.
Heat olive oil in a large skillet over med-high. Add onion and cook, stirring occasionally, until soft, about 4 minutes. Add the garlic and cook, stirring, 30 seconds.
Add the sausage and cook, breaking it up with a wooden spoon, until browned, about 5 minutes.
Add the spinach, 1/2 tsp salt, and pepper to taste and stir until heated through, about 2 more minutes. Remove from the heat and let cool completely.
Combine the spinach mixture, ricotta, 1 cup mozzarella, the Parmesan, parsley and 1 tsp salt in bowl. Stuff each shell with about 2 T of the filling and set aside.
Preheat oven to 350.