• Preheat the oven to 375º.
• See how many pasta shells will fit in your large baking dish and cook them according to the package directions. Drain and rinse under cold water. Set them on a towel and dry them as much as the water as possible.
• In a large skillet over medium heat, add the olive oil. Stir until the onions are translucent, about 10 minutes. Add the finely chopped parsley,
• Add the ground pork and ground beef, and cook until cooked through, breaking into small pieces as it cooks.
• Mix in the chopped spinach, cook until wilted, and no more water is released. Transfer the mixture to a large flat plate or serving dish and cool it completely. I placed mine on a small cookie sheet, spread it out, and refrigerated it to cool.
• In a large bowl, add the ricotta, egg, ¼ of the parmesan cheese, ¾ of the mozzarella cheese, salt, and pepper. Mix well, then, once the meat has cooled, add it to the mixture and combine until fully incorporated.

Bechamel Preparation

• In a large heavy-bottomed pan over medium heat, melt butter and add the flour. With a whisk, mix until a thick cream forms. Add slowly the milk, constantly whisking, making sure no lumps are formed.
• Add salt, pepper, and grated nutmeg as needed. The sauce should leave a thin layer of sauce on the back of a wooden spoon when it is ready.

Assembling

• Using a small spoon or a piping bag, fill each shell with the meat and cheese mixture until full.
• Pour the tomato sauce into the bottom of a large oven-proof dish.
• Add 1/2 of the bechamel over the tomato sauce; there's no need to blend it with the tomato.
• Add your stuffed shells over the sauce. Sprinkle with the remaining Parmesan cheese and shredded mozzarella , then bake at 375 F for 30 minutes, or until the top is browned.
You can make this the day ahead