Season both sides of the chicken with the umami seasoning, 1 teaspoon chicken seasoning and approximately half of the black pepper.
Heat the oil and 1 tablespoon of butter in a large skillet over medium heat.
Add the chicken and cook for approximately 4–5 minutes per side, or until browned and cooked through to an internal temperature of 165°F (74°C). Transfer the chicken to a plate and loosely tent with foil.
Add the remaining 2 tablespoons of butter to the same skillet. Add the onion and mushrooms, season with the remaining black pepper and sauté for about 5 minutes.
Add the garlic and cook for 1 minute.
Sprinkle the flour over the mushroom mixture. Stir well and cook for about 2 minutes
Slowly stir in the chicken broth, scraping the browned bits from the bottom of the skillet.
Stir in the heavy cream, thyme, Worcestershire sauce and ¼ teaspoon chicken seasoning.
Bring to a gentle simmer and cook for 3–5 minutes, stirring occasionally, until the sauce has thickened.
Reduce the heat to low. Add the Parmesan and stir until completely melted.
Stir in the fresh parsley. Taste the sauce and add salt only if needed.
Serve the chicken over garlic mashed potatoes, if desired, and spoon the creamy mushroom sauce generously over the top. Garnish with additional fresh parsley.