Dissolve the cornstarch in the vegetable broth and set aside.

Heat a large cast iron pan over medium-high heat. Sauté the seitan in 2 tablespoons of olive oil until lightly browned, about 5 minutes. Transfer to a plate and set aside. Lower the heat to medium.

Add the remaining 2 tablespoons of olive oil and the shallots. Sauté for 5 minutes until translucent. Add the garlic, cremini and portobello mushrooms, and thyme. Sauté for about 10 minutes, until the mushrooms have browned and released their moisture.

Meanwhile, bring a large pot of lightly salted water to a boil. Cook the fettuccine according to package directions.

Add the salt, wine, and paprika to the mushroom mixture. Turn the heat up to high and reduce the liquid, about 10 minutes.

Add the cornstarch mixture, stir well, and let the sauce thicken, about 7 minutes. Add the nutritional yeast, milk, and mustard. Mix well and lower the heat to low — do not let it boil at this point as it can make the milk and mustard bitter. Add the seitan and peas and cook for 5 more minutes.

Divide the noodles among bowls and top with the stroganoff. Serve immediately so the pasta doesn't stick. Garnish with fresh parsley if you like, and a dollop of vegan sour cream if you want to lean into the full stroganoff experience.