In a large wide heavy skillet or sauté pan, melt the butter over medium heat. Add the chopped shallots and cook for about 2-3 minutes, or until they are softened. Then, add the sliced mushrooms and cook until the moisture evaporates and the mushrooms become tender, which should take about 10-12 minutes while stirring occasionally. Season the mushrooms with salt and pepper after they have released their moisture.
Stir in the minced garlic and Italian seasoning, cooking until fragrant, which should take about 30 seconds while stirring constantly. Sprinkle the flour over the mushrooms and cook for about 1 minute while stirring constantly.
Pour in the chicken broth and bring the mixture to a simmer. Scrape the bottom of the pan to release any browned bits.
Add the half and half and soy sauce to the pan. Season with salt and pepper, then let the mixture simmer until it thickens, which should take about 4-5 minutes.
Stir in the freshly grated Parmesan cheese and chopped parsley. Taste the sauce for seasoning and adjust with salt and pepper as needed. For a brighter flavor, you may add the juice of half a lemon, though this step is optional.
Cook the fettuccine in salted water until al dente according to the package directions. Just before draining, reserve some of the pasta water. Toss the cooked pasta with the sauce, and if needed, thin it out with the reserved pasta water. Serve with additional Parmesan cheese.
