Preheat the oven to 350 degrees.
Bring a pot of water to boil. Sprinkle in some salt, then cook the tortellini according to the package instructions (usually only have to boil between 5 to 10 minutes). Drain.
In a large skillet, melt the 1 tbsp of butter over medium-low heat.
Once melted, sprinkle in the flour and whisk to combine.
Slowly pour in the 1 cup of milk as you continually whisk. This will form a thin white sauce (also known as a "roux").
Sprinkle in the parmesan cheese. Whisk to combine.
Add in the cooked chicken, spinach, tortellini, and 1/4 cup of the mozzarella. Stir to combine.
Grease an 8x8" casserole dish with some butter.
Scoop in the chicken tortellini mixture.
Sprinkle the top with the other 1/4 cup of mozzarella cheese and the Italian seasoning.
Bake in the oven for 15 minutes.
Switch the oven to broil and broil for 2-3 minutes, or until the mozzarella becomes golden brown on top.