Bring a large pot of salted water to a boil. Add the 8 oz of fettuccine pasta and cook according to package instructions until al dente, usually about 10-12 minutes. Drain the pasta, reserving about 1/2 cup of pasta water, and set aside.
Heat 2 tablespoons of olive oil in a large skillet over medium-high heat. Season the 1 pound of sliced chicken breasts with 1 teaspoon salt and 1/2 teaspoon black pepper. Add the chicken slices to the hot skillet and cook for 4-5 minutes on each side until golden brown and cooked through. Remove the chicken from the pan and set aside.
In the same skillet, add the sliced mushrooms and sauté for about 5 minutes, stirring occasionally, until they release their moisture and begin to brown. Add the minced garlic and cook for an additional 1-2 minutes until fragrant.
Pour in 1 cup of chicken broth and 1 tablespoon of balsamic vinegar to the skillet with the mushrooms and garlic. Stir to combine and bring the mixture to a simmer. In a small bowl, whisk together 1 tablespoon cornstarch and 1/4 cup water to create a slurry, then slowly pour it into the skillet while stirring. Continue to simmer the sauce for 2-3 minutes until it thickens.
Return the cooked chicken to the skillet, stirring to coat it well in the sauce. Add the drained fettuccine to the skillet, tossing everything together gently. If the sauce seems too thick, add a splash of the reserved pasta water to loosen it up.
Turn off the heat and sprinkle 1/4 cup of freshly chopped parsley over the pasta. Serve immediately with a generous amount of grated Parmesan cheese on top.
