Preheat the oven to 375°F.
Thaw the frozen spinach in the microwave. Once it is cool enough to handle, squeeze out the excess water and set aside. I use a cheesecloth to wring the water out.
Heat a pan over medium high heat. Add olive oil to coat the bottom. Add the onions, red bell pepper, garlic and dried thyme and saute until the vegetables are soft, about 7 minutes.
Add the chopped spinach. and mix until it is well incorporated. Season with salt and pepper and saute for a few minutes. Then, set the mixture aside to cool.
In a large mixing bowl, add the ricotta cheese, shredded cheese, parsley, egg, salt and pepper and mix until well combined.
Add the vegetable mixture to the ricotta cheese mixture and mix until well combined. Set aside.
Then, bring a pot of salted water to a boil and cook the shells, per package instructions until the shells reach al dente. Then, strain them, put them back in the pot, drizzle with olive oil and give them a good toss. This prevents them from sticking. Then, set them aside to cool.
When the shells are cool enough to handle, you are ready to assemble. Spoon enough Arrabiata sauce to coat the bottom of an oven safe baking dish. Add a tablespoon of filling to each of the shells and lay them filling side up in the baking dish. Continue until all the shells are filled.
When the baking dish is full, top with more arrabiata sauce, and a generous layer of shredded cheese. Put the dish in the oven for 35-45 minutes or until the top is golden and bubbling.
Serve warm from the oven with a green salad and some crusty french bread.
