Heat the oil over medium heat. Once hot, add the mushroom and onions. Cook, stirring occasionally, until the onions are translucent and the mushrooms have browned. Stir in the garlic and cook a minute or so, until fragrant. Remove all that to a plate and set aside.
Add the ground beef to the skillet and cook until browned, crumbling it up with a wooden spoon as you cook it. Drain any excess grease, then return the mushroom mixture to the skillet. Increase the heat to med-high.
Stir in the uncooked pasta, the milk and the beef broth to the skillet. Stir in the seasonings. For the cornstarch, create a slurry by whisking it into 2 tbsp. of the broth in a small bowl, then stir that in. Once it all reaches a boil, reduce the heat to med-low and cover with a lid.
Simmer, stirring occasionally, for 10-12 minutes, until the noodles are al dente.
Remove from heat and stir in the sour cream. Let sit for 5 minutes. Then serve garnished with freshly chopped parsley. Refrigerate any leftovers.
