01 -
Heat your oven to 375°F. Follow the directions on the pasta package to cook shells until slightly firm (al dente).
02 -
Warm the olive oil in a wide pan, soften the onion (around 3-4 minutes), then toss in the garlic and sauté for 60 seconds more.
03 -
Put the spinach into the pan and cook briefly, letting the leaves soften just enough (2 minutes).
04 -
Mix the cooked spinach, cheeses, egg, salt, and pepper in a big bowl until evenly combined.
05 -
Pour half of the marinara sauce onto the bottom of an 8x8 or 9x9 baking pan.
06 -
Spoon the cheese-spinach mixture into each shell, then place the filled shells neatly in the prepared baking dish.
07 -
Spread the rest of the marinara over the shells and bake for 20-30 minutes. They should heat all the way through.
08 -
Sprinkle some fresh basil and extra Parmesan on top before serving warm.