Preheat the oven to 350°F. Spray a 13 x 9 inch baking dish with cooking spray, Set aside.
Cook the pasta shells in salted water per the package instructions until al dente. Drain well.
In a large pot over medium-high heat cook the diced onion in a few drizzles of olive oil for 2 minutes until softened. Add the ground beef. (Tip: Using a large pot doubles as the mixing bowl)
Season the beef with Italian seasoning, garlic salt and red pepper flakes. Continue to cook the beef until no pink remains. Drain all excess fat from the pot.
To the ground beef add the cream cheese, half and half and butter. Stir until the cream cheese has completely melted.
Remove the pot from the heat and add 2 cups shredded cheese, pesto, sundried tomatoes , Parmesan and cooked pasta. Mix well.
Pour 1/2 of the mixture into the bottom of the baking dish and sprinkle with 1 cup of shredded cheese. Top with the remaining pasta and 1 cup shredded cheese.
Bake for 40 minutes or until golden and bubbly. Check at 30 minutes and cover with foil, if needed.
Let stand on the counter for 5 minutes then garnish with chopped parsley just,before serving.