Bring a large pot of generously salted water to a boil. Cook the linguine until al dente according to the package directions, reserve about 1/2 cup of pasta water, and drain.

Season the bite-size chicken strips with about 1/2 teaspoon of the kosher salt and the black pepper. Heat the olive oil in a large skillet over medium-high heat.

Add the chicken and cook for approximately 4–6 minutes, turning occasionally, until browned and cooked through. Transfer the chicken to a plate.

Add the sliced mushrooms and chopped onion to the same skillet. Cook for 5–7 minutes, until the mushrooms release their moisture and begin to brown.

Stir in the minced garlic and dried thyme, if using. Cook for about 30 seconds, just until the garlic becomes fragrant.

Pour in the chicken broth and scrape up the browned bits from the bottom of the skillet. Simmer for 1–2 minutes so the broth picks up the savory flavor in the pan.

Reduce the heat to medium-low, then stir in the heavy cream and Dijon mustard. Simmer gently for 2–3 minutes, avoiding a vigorous boil so the creamy sauce stays smooth.

Return the cooked chicken to the skillet and add the drained linguine. Toss until the pasta is coated, adding a little reserved pasta water if the sauce is too thick.

Taste and adjust the salt and pepper as needed. Finish with chopped parsley and grated Parmesan, if desired, and serve immediately.