Preheat oven to 375 degrees F.
Grease 9" baking dish with cooking spray.
In a large skillet or pot, add the olive oil over medium-high heat.
Add the onion, red bell pepper, and green bell pepper, and cook until softened, about 5-7 minutes.
Add the garlic and Italian seasoning, and cook one more minute.
Add the butter and cook until it has melted.
Add the flour and stir well to combine. Continue to cook for 1 minute.
Slowly add the chicken broth, followed by the half and half, stirring continuously to create a creamy sauce.
Stir in the diced tomatoes, Parmesan, and 1/2 cup Cheddar cheese, and stir to combine.
Stir in the shredded chicken and spinach, and stir until spinach has wilted.
Lastly, stir in the al dente spaghetti to combine.
Transfer mixture to prepared baking dish and top with remaining Cheddar cheese.
At this point you can cover with plastic wrap and freeze, refrigerate until ready to bake, or bake immediately.
If baking immediately, place in oven and bake for 20-25 minutes, until cheese is melted and bubbling.
If baking from refrigerator, bake for 40-45 minutes, until cheese is melted and bubbling.
If baking from the freezer, bake 45 minutes covered with foil, and then an additional 20-30 minutes with foil removed, until cheese is melted and bubbling.
Serve and enjoy!