Boil a pot of water for the egg noodles and cook according to package directions (or make rice or mashed potatoes or whatever else you want to serve with this recipe).
Add the oil and butter to a skillet over medium-high heat. Once the butter has melted, add the onions and sauté for 5 minutes.
Stir in the mushrooms and cook for another 7-8 minutes or until they release their water, the water cooks off, and they start to lightly brown.
Add the garlic and flour to the skillet. Give it a good stir and cook for 1 minute.
Stir in the vegetable broth, Worcestershire sauce, and Dijon mustard. Ensure the flour has dissolved, and let it cook for a couple minutes until the sauce thickens a bit.
Turn the heat off and stir in the sour cream. Toss with the drained noodles and season with salt & pepper as needed.
Notes
Makes 4 reasonably sized portions. If you're feeding very hungry people, you may want to have an appetizer or side dish.
Worcestershire sauce isn't vegetarian, but some brands do sell vegetarian versions.