Prepare egg noodles according to package directions.
Heat oil in a large nonstick skillet over medium-high heat. Add mushrooms and onions; cook, stirring occasionally until mushrooms are browned and the onions are translucent, about 8 minutes.
Add garlic and butter; cook, stirring, for 30 seconds or until butter has melted.
Add flour, mustard, and salt and pepper; stir until mushrooms are coated.
Slowly add vegetable broth, stirring until sauce thickens, about 4-5 minutes.
Remove from heat and stir in sour cream.
Add drained egg noodles and toss to coat.
Garnish with parsley, if desired.
