Yield6-8 servings
Prep Time37 minutes
Cook Time38 minutes
Total Time75 minutes
Preheat the oven to 375°F (190°C).
Half-fill a large pot with water, and bring to a boil over high heat. Add pasta shells, and cook al dente, about 8 minutes, stirring regularly. Drain the cooked shells and run them under cool water to stop them from cooking further.
While the pasta is cooking, place the chopped fresh spinach in a mesh sieve, and balance the sieve at the top of the pasta pot. Cover the pot with a lid, removing occasionally to stir the pasta. Remove and drain the spinach when it’s bright green and wilted, about 3-4 minutes. (Depending on the size of your sieve, you may need to steam the spinach in two batches. If you don’t have a sieve, you can wilt the spinach in a large skillet with water over medium heat.)
In a medium mixing bowl, mix ricotta, flour, onion powder, black pepper, spinach, and mozzarella (if using).
Spread 2 1/2 cups marinara sauce on the bottom of a large 10 x 15 baking dish.
Fill each pasta shell with a large spoonful of spinach ricotta filling and place in the prepared baking dish. You can also use a piping bag with a large round tip to fill the shells, if you prefer. When all shells are in the dish, top the shells with an additional 1 cup of marinara sauce.
Cover the baking dish with aluminum foil and bake for 20 minutes at 375°F (190°C). Remove the foil and bake an additional 10 minutes.
This recipe makes enough filling for 21-24 shells (depending on whether you use mozzarella); but a 12-oz box contains 36-40 shells. Some shells may tear, so you may wish to cook more than 24 shells, but you don't need to cook the whole box.
If you do want to make use of all the shells in a 12-oz. box, multiply the ingredients by 1.5x. You will likely need an additional casserole pan to fit all the shells.
Cooking the shells al dente reduces tearing, and the shells will soften further in the oven. You don't need to cook the shells for the time listed on the box.
Store and reheat: Cover casserole dish with plastic wrap or place shells in an airtight container and refrigerate for up to 3-5 days. Reheat in the oven at 350°F (180°C) for 15 minutes.