Thaw out the spinach. Put the thawed spinach into a mesh strainer, flatten it out so you squeeze as much of the water out of the spinach as possible. Sprinkle with a pinch or two of kosher salt and remix (it is important to get some of the water out of the spinach, so the cheese filling doesn't become soggy).

Cook the manicotti shells in well-salted water. I like to stop them a bit before they are al dente, so they will not tear or shred when being handled.

Now mix up the cheese filling. To season the cheese up, mix the ricotta cheese in a large mixing bowl with the eggs, the Pecorino-Romano, two cups of the shredded mozzarella, the garlic powder, parsley, oregano, pepper, and finally the spinach. Don't add salt here, the filling will pick up plenty of saltiness from the prosciutto. Mix well by hand until uniformly mixed.

Next, prep the prosciutto-cheese roll ups. Spoon a generous 1/3 cup of the cheese filling unto each slice of prosciutto and roll up the prosciutto width wise. Don't be scared to roll the prosciutto somewhat tightly, the cheese will come out towards the edges of the prosciutto but it will keep together well enough to not fall off apart.

Once each slice of prosciutto has been rolled up with the cheese, place one prosciutto-cheese roll up into each manicotti shell (you can cheat here by cutting the manicotti lengthwise, placing the roll up inside, and then re-closing the manicotti shell).

Use some butter to grease up the bottom of two 9" by 13" oven safe dishes or baking tins, then put a generous bottom layer of the marinara sauce. You need two baking dishes so the manicotti is not too crowded.

Put each stuffed manicotti shell onto the dish, one shell thick only. Cover with the remaining marinara sauce and mozzarella cheese and cover with foil.

Bake at 350 degrees for 30 minutes. Remove the foil and cook for 20-30 minutes more, or until the cheese is melted and slightly bubbly. Remove from the oven, garnish with some parsley, and serve while hot.