Season chicken with salt, pepper, garlic powder, and onion powder.
Heat the olive oil in a large skillet over medium-high heat.
Once the pan and the oil are hot, add the chicken. Allow both sides to sear for 3-5 minutes. Time will depend on how thickly cut the chicken is. The internal temperature should read 165 degrees F in the thickest part of the chicken using a meat thermometer.
Remove the chicken to a plate; set aside and cover to keep warm.
If the pan appears dry, add 1 additional tablespoon of olive oil and allow to heat.To the skillet, add the sliced mushrooms. Allow to saute for 3-4 minutes. Water will naturally release. Saute until no water remains.
Using a slotted spoon, remove the mushrooms to the same plate as the chicken and cover to keep warm.
To make the sauce, add the butter to the skillet to melt. Once the butter is just starting to bubble, add the minced garlic. Saute for 30 seconds and then whisk in the flour to thicken.
Whisk in the chicken stock, half and half, and the Italian seasoning. If using unsalted stock/broth, season with salt and pepper, to taste. Using a wooden spoon or spatula, stir well, making sure to scrape any bits off of the skillet as the stock heats.
Allow to simmer for 2-3 minutes before adding the parmesan cheese, stirring until melted and the sauce has begun to thicken slightly. Add the chopped spinach and cook until it starts to wilt.
Add the chicken and mushrooms back to the sauce to warm for 1-2 minutes. Remove from heat.
Garnish with freshly chopped parsley and serve over mashed potatoes, quinoa, rice, or your favorite pasta!