Melt 2 Tbsp of the vegan butter and then add in the diced onions / shallots. Cook for 4 - 5 minutes or until slightly translucent and softened.
Add in the mushrooms and cook down for about 6 - 8 minutes or until they've browned. Add in the garlic and cook for one minute.
Add the flour and toss to coat the flour on the mushrooms and onions. Cook for 1 minute and add in the vegan beef or veggie broth and stir.
Add in the dijon mustard, vegan worcestershire, nutritional yeast, and sour cream and stir to combine. Taste for seasoning and simmer until the sauce is thickened to your liking. Add in more veggie broth if it's too thick.
Toss the reserved soy curls back into the gravy and stir to combine. I've noticed that the soy curls will absorb the gravy so don't cook them in the gravy for too long. Add in more veggie broth if needed and serve immediately over mashed potatoes or pasta.
You can also stir the pasta straight into the stroganoff as well. If sauce is too thick, thin out with some veggie broth or pasta water. If it's too thin, you can make a quick slurry with 1 tsp of cornstarch or potato starch and 1 Tbsp of water. Whisk and then add in as much as you need to thicken the sauce.