If using chicken breasts (or large thighs), place the chicken in a plastic zip-top bag and pound to an even thickness.
Season with salt and pepper.
Heat the oil over medium-high heat.
Add the chicken to the pan and cook for 7 minutes per side, or until the internal temperature is 155° F.
Remove the chicken from the pan and tent with foil.
Add mushrooms and shallots and cook for 7 minutes.
Add the spinach and garlic and cook for about 1 more minute.
Add the broth or wine to the pan and scrape up any brown bits.
Reduce heat to medium-low and add the heavy cream and parmesan cheese.
Cook for 3 to 5 minutes until the cheese melts.
Return the chicken to the pan and toss to coat. Cook for another two minutes until the chicken reaches 165° F.