Heat olive oil in a large deep skillet or pot over medium-high heat.
Season the chicken pieces with salt and black pepper.
Add the chicken to the skillet and cook for 5-6 minutes, stirring occasionally, until browned and cooked through. Remove chicken from the skillet and set aside.
In the same skillet, add the sliced mushrooms and cook for 4-5 minutes until they release their moisture and begin to brown.
Add the minced garlic to the mushrooms and cook for 1 minute until fragrant.
Pour in the chicken broth and bring to a simmer, scraping up any browned bits from the bottom of the skillet.
Reduce heat to medium-low and stir in the heavy cream, Parmesan cheese, dried thyme, and crushed red pepper flakes. Stir until the cheese melts and the sauce thickens slightly, about 3-4 minutes.
Add the fresh baby spinach to the sauce and cook for 2-3 minutes until wilted.
Return the cooked chicken to the skillet and stir to coat with the creamy sauce. Cook for an additional 2 minutes to heat through.
Taste and adjust seasoning with additional salt and pepper if needed.
Serve hot, spooning the creamy chicken mushroom Florentine over rice, pasta, or crusty bread if desired.