For the chicken breasts: Place the chicken breasts between two pieces of plastic wrap and pound to an even thickness using a meat mallet.
Mix together the salt, pepper and flour in a bowl, then transfer to a shallow pan for dredging. Dredge the chicken breasts in the seasoned flour right before you put them in the pan.
Heat a saute pan over high heat. When it's good and hot, add the grapeseed oil and then the dredged chicken breasts (do not over crowd the pan, cook in batches if needed). Cook until the chicken is golden brown and seared on one side, 3 to 5 minutes. Flip and cook on the reverse side until cooked through, another 3 to 5 minutes depending on thickness. Remove the chicken from the pan and let it rest a few minutes before plating.
For the Marsala sauce: Heat a large saute pan over high heat. Add the grapeseed oil, then the garlic. Cook for about 30 seconds before adding the sliced cremini, oyster and shiitake mushrooms. Saute until tender. Add the rosemary and sprinkle with salt and pepper. Deglaze the pan with the Marsala and cook until the wine is reduced by half. Add the demi-glace, bring to a simmer and cook for 2 minutes. Remove the sauce from the heat and swirl in the butter to incorporate it. Season with salt and pepper.
Add the chicken to the pan and coat with the sauce. Transfer the chicken to serving plates and divide the sauce between the plates, spooning the mushrooms and sauce over the breasts. Garnish with chopped parsley.