Bring a pot of heavily salted water to a boil. Cook the tortellini according to package instructions. Drain, and set aside.
While waiting for the water to boil, in a large skillet over medium heat, cook the sliced sausages until cooked and nicely browned. Transfer to a plate or bowl.
Add the butter to the drippings, stirring to melt. Stir in the garlic, Italian seasoning, and red pepper flakes until fragrant, about a minute.
Stir in the flour, constantly whisking until slightly browned and the flour taste has cooked out, about a minute or so. A little at a time, whisking constantly-add in the milk and cream, waiting each time until the roux has reformed before adding some more, until it's all come together.
Let the mixture simmer, whisking occasionally, until thickened, about 3-5 minutes.
Remove the skillet from heat and stir in the cheese. When the cheese has melted, add the sausage & pasta to the skillet. Stir until everything's evenly combined. Salt & pepper, to taste.