Preheat oven to 375°F. Prepare an 8X8 baking dish. Set aside.
Cook shell pasta until al dente, according to package instructions. Drain and set aside.
In a small skillet, cook Italian sausage until browned and crumbled, about 5-7 minutes. Drain and set aside.
In a large skillet, heat olive oil over medium-high heat. Add garlic and cook until fragrant, about 1-2 minutes. Stir in spinach. Cook until it starts to wilt, about 3-4 minutes. Remove from heat and let cool.
In a mixing bowl, stir together spinach, ricotta, mozzarella, Parmesan, egg, basil, Italian seasoning, Kosher salt, and pepper until well combined. Pour ½ cup of marinara sauce into the bottom of an 8X8 baking dish. Layer Italian sausage crumbles evenly on top of sauce.
Stuff each pasta shell with ricotta mixture and place in baking dish.
Pour the remaining sauce and cover the baking dish with aluminum foil. Place into oven.
Bake at 375°F for 25 minutes. Remove aluminum foil. Return to oven to bake for an additional 10-15 minutes, until top begins to brown.
Serve immediately warm and top it with Parmesan cheese.