Cook the pasta according to the box instructions. Reserve 1 ½ cups of pasta water, then drain the pasta and set it aside. You'll need about 1 cup; a little extra is always a great idea.
Sauté:
In a large skillet, heat the olive oil and butter over medium-high heat. Add the chopped onion and sauté until it has softened, about 3 minutes. Add the garlic, mushrooms, and chopped walnuts (if using) and stir.
Add the seasonings to the mushroom mixture, stir to combine, and continue cooking the mushrooms for 8 to 10 minutes, until they've turned a darker color and softened. You'll see some of the mushroom moisture being released in the pan.
Sprinkle the cornstarch over the mushrooms and toss to distribute it. Add 1 cup of the reserved pasta water, stir, and bring the sauce to a low simmering boil.
Reduce the heat to the lowest setting and simmer the sauce for 5 minutes.
Combine:
Add the cooked pasta to the mushroom sauce. Sprinkle the Parmesan cheese over the top, and toss to combine. If you want this to be more saucy, add a little more reserved pasta water to the skillet.
Serve warm, topped with a little more Parmesan.
Notes
Use 1lb (16 ounces) of your favorite mushrooms. Simple button mushrooms, Shitake, chopped Portabello, anything goes.
Other herbs that work well are Italian seasoning, poultry seasoning, oregano, thyme, and rosemary. Use 1 teaspoon to 1 ½ teaspoons of dried herbs, and 2 to 3 teaspoons of finely chopped fresh herbs.