Slice chicken lenthwise to form cutlets. Season both sides with salt and pepper (about 1/2 teaspoon salt and 1/4 teaspoon black pepper).
Dredge chicken flour shaking off excess and set aside.
To a large skillet, add buter and oil over medium to medium-high heat. sear chicken (about 2 minutes per side) then remove from the skillet to a plate and set aside.
Add mushrooms, onions and minced thyme to the skillet (don't wash it) cooking 6-7 minutes or until golden then stir in minced garlic and Worcestershire sauce.
Pour in the wine, scraping any browned bits from the bottom of the pan and cook 2-3 minutes letting the wine reduce.
Add chicken stock, parmesan cheese and cream. Once it comes to a boil, reduce heat to medium-low and simmer 2-3 minutes until the cheese is melted and the sauce thickens.
Whisk in the dijon mustard, combining well then return the chicken pieces to the skillet, turning to coat in the sauce.
Wilt in chopped spinach and simmer 1 minute or until the chicken is warmed and an internal temperature of 165 degrees on a meat thermometer.
Taste and adjust for salt and pepper then serve right away.