Cook pasta in a pot of salted water until just shy of al-dente.
Drain, and set aside.
In a pan, cook sausage until brown over medium heat breaking it up into small pieces with your spatula.
Remove sausage on a plate, and drain fat, leaving 2 tbsp in the pan. Set aside the fat for later.
In the same pan, over medium high heat, cook garlic for 30 seconds.
Then add mushrooms, season with salt and pepper, and saute until brown.
Pour mushrooms onto the same plate where you have your cooked sausage.
Add 2 tbsp of reserved fat into the same pan, and stir in flour.
Cook over medium high heat for a minute.
Then slowly drizzle in milk while stirring consistently with a whisk or wooden spoon until combined, and sauce thickens.
Add in cooked pasta, sausage, and mushrooms.
Toss to combine, and season with salt and black pepper to taste.
Remove from heat.
Toss in chopped parsley before serving.