01 -
Preheat oven to 375°F and cook pasta shells until al dente following package instructions
02 -
Heat olive oil in a large skillet, cook diced onion until tender (3-4 minutes), then add minced garlic and cook for one more minute
03 -
Add spinach to the skillet and cook until just wilted, about 2 minutes
04 -
In a large bowl, combine cooked spinach mixture with ricotta, mozzarella, Parmesan, egg, salt, and pepper
05 -
Spread half of the marinara sauce in the bottom of a 8x8 or 9x9 baking dish
06 -
Fill each shell with the spinach-cheese mixture and arrange in the prepared baking dish
07 -
Top with remaining sauce and bake for 20-30 minutes until heated through
08 -
Garnish with fresh basil and additional Parmesan cheese if desired, and serve immediately