01 -
Set your oven to 375°F and cook the pasta shells following the package instructions until they're just tender.
02 -
Warm up olive oil in a big pan. Throw in the diced onion, cooking it until it softens (takes 3-4 minutes). Add in the garlic and let it cook for another minute.
03 -
Toss spinach into the skillet and stir until it wilts down, which should take about 2 minutes.
04 -
Stir together the cooked spinach, ricotta, mozzarella, Parmesan, egg, salt, and pepper in a large bowl until it's all well combined.
05 -
Pour about half the marinara sauce into the base of an 8x8 or 9x9 dish, spreading it out evenly.
06 -
Stuff each pasta shell full of the spinach and cheese mixture. Arrange them neatly into the sauced dish.
07 -
Spoon over the rest of the marinara sauce, then pop it in the oven for 20 to 30 minutes until it's completely heated through.
08 -
Sprinkle with extra Parmesan and a bit of fresh basil if you'd like, then enjoy right away.