Preheat the oven to 350°F. Spray a 13 x 9 inch glass baking dish with cooking spray.
Cook the shells per the instructions on the box in salted water until al dente, then drain and cool slightly.
In a stove top pan, saute the chopped onion over medium high heat in a couple of tablespoons of olive oil, until translucent. Add the ground beef to the pan and cook until no pink remains. Season with salt and black pepper. Drain excess fat from the pan.
To the pan add the sun dried tomatoes, pesto, minced garlic, Italian seasoning, and red pepper flakes to the ground beef. Stir until combined.
Add the whipped cream cheese, ricotta cheese and spinach. Simmer on low heat for 2 minutes or just until the cream cheese has melted. Taste and adjust salt and black pepper to your taste. Remove from the heat.
Next, add the egg, 1 cup shredded Italian cheese and 1/3 cup of grated Parmesan cheese. Stir thoroughly.
Fill shells using a tablespoon then place side by side, seam side up in the dish. Drizzle the tops with the marinara sauce.
Bake for 35-40 minutes or until heated through and bubbly. Place the remaining shredded cheese on top five minutes before the end of baking.
Serve immediately garnished with parsley.