Boil the half pound of pasta in salted water according to package directions. Remember to save some pasta water right before draining and set aside.
Drain the pasta in a colander and rinse very well with cold water to prevent it from becoming mushy.
Set the same large pot that you boiled the pasta in on the stove over medium heat. Add 2 tablespoons of the butter and the diced onion. Cook 3 to 4 minutes, stirring a few times.
Add the minced garlic, fresh spinach, kosher salt, pepper, and crushed red pepper flakes. Cook this 3 to 4 more minutes, stirring a few times, and remove from the pot and set aside.
Put the same pot, still over medium heat, and add the remaining 2 tablespoons of butter, the heavy cream, Italian seasoning, kosher salt, pepper, and crushed red pepper flakes.
Bring to a boil and then reduce the heat to medium low and simmer for 5 to 7 minutes stirring a few times. Stir in the shredded parmesan cheese during the last minute or so.Note: Carefully watch as the cream comes to a boil, remove the pot from the heat for a moment if it bubbles up too much. Continue stirring until it reaches a gentle simmer. Add the shredded chicken, sun dried tomatoes, cooked pasta, the reserved pasta water that was set aside earlier, and cooked spinach and stir until combined.
Taste for seasoning and add more kosher salt, pepper, crushed red pepper flakes, and/or Italian seasoning if needed. Transfer to a casserole dish, top with the shredded mozzarella cheese.
Set the smoker or pellet grill to 225. If you have "Super Smoke" as a feature, turn that on. Place the casserole dish on the grate for an hour.
Serve garnished with basil or parsley if desired.