Preheat the oven to 425°F and bring a large pot of salted water to a boil.
Cook the jumbo pasta shells for 6 minutes until al dente. Drain, rinse under cold water, and toss with a little olive oil to prevent sticking.
In a medium bowl, combine finely chopped spinach, ricotta cheese, half the mozzarella, half the parmesan, the egg, Italian seasoning, SPG, and red pepper flakes. Mix well to form the cheese mixture.
Spread tomato sauce or homemade sauce on the bottom of a baking dish.
Fill each shell with about a tablespoon of the ricotta mixture and place in the dish.
Spoon a little more pasta sauce over the top of the shells and sprinkle with the remaining cheese.
Cover with aluminum foil and bake for 20 minutes. Remove the foil and bake another 5 minutes until bubbly and golden brown.
Notes
Make-Ahead Option: Assemble them up to a day ahead, cover with aluminum foil, and refrigerate. Just add 10 minutes to the bake time when ready to cook.
Freezer Instructions: Stuff and sauce the shells but skip baking. Freeze in a casserole dish, then bake straight from frozen at 375°F until heated through (about 45–50 minutes).
Reheating Instructions: Reheat in a large skillet over medium heat with a splash of pasta sauce, or cover and bake at 350°F until hot.