Position an oven rack to the middle position.
Preheat the oven to 400 degrees F.
In a large pot of salted, boiling water, add the pasta and cook just to al dente, according to the package directions.
Drain the pasta.
Add the extra olive oil to the cooked pasta and toss to coat. Set it aside.
In a food processor, add the bread slices and cover the processor.
Pulse until the bread becomes coarse crumbs.
In a large mixing bowl, add 3/4 cup of the breadcrumbs, the ground turkey, the turkey sausage, the egg, the salt, the parmesan cheese, and 1/4 cup of the parsley and with your clean fingers or two forks, mix lightly until the ingredients are just combined. Be careful not to compact the meat.
Form the meat mixture into 2-inch balls.
In a large, deep, ovenproof skillet over medium-high, add 1 1/2 tablespoons of the remaining olive oil.
Add the meatballs to the hot oil and cook, while turning them occasionally, until they are browned on all sides, about 10 minutes.
Using a slotted spoon, transfer the meatballs to a plate.
Add 1/2 tablespoon of the remaining olive oil to the skillet and allow it to heat.
Add the onions to the hot oil and cook, while stirring, until they begin to brown and soften, about 4 minutes.
Add the garlic, the oregano, and the red pepper flakes to the onions and cook, while stirring, until just fragrant, about 30 seconds.
Add the tomato sauce and the spinach to the onion mixture, stirring to combine.
Add the meatballs to the sauce mixture and let them simmer until they reach an internal temperature of 165 degrees F, about 20 minutes.
Add the pasta and 4 ounces of the shredded mozzarella to the sauce mixture and gently stir to combine.
Top the meatball mixture with the remaining sliced mozzarella.
Bake the meatball mixture on the middle rack until the cheese is melted and lightly crisp on top and the edges are bubbling, about 15-20 minutes.
Allow the casserole to rest for 5 minutes.
Serve garnished with the extra parsley.