Bring a large pot of salted water to a boil. Add the fettuccine or egg noodles and cook according to package instructions until al dente. Drain and set aside, reserving a little pasta water for later.
Heat olive oil in a large skillet over medium heat. Add diced onions and sauté until they become translucent and soft, about 4-5 minutes. Stir in the minced garlic and cook for another 30 seconds until fragrant.
Add sliced button mushrooms to the skillet. Cook, stirring occasionally, until the mushrooms release their moisture and become golden brown, roughly 6-8 minutes.
Pour in the vegetable broth, stirring to deglaze the pan and lift all those lovely browned bits from the bottom. Let it simmer for 3-4 minutes to reduce slightly. Then stir in the heavy cream, Dijon mustard, and soy sauce. Simmer gently until the sauce thickens, about 5 minutes.
Add the cooked pasta to the skillet, tossing gently to coat every strand in the creamy mushroom sauce. If the sauce feels too thick, add a splash of the reserved pasta water to loosen it up.
Season with salt and pepper to taste, then sprinkle with the fresh chopped parsley. Serve immediately and enjoy the comforting, creamy goodness.