Season the chicken breasts with salt and pepper on both sides.
In a large skillet, heat the olive oil over medium-high heat.
Add the chicken breasts to the skillet and cook for about 6-7 minutes per side, or until golden brown and cooked through. Remove the chicken from the skillet and set aside.
In the same skillet, add the minced garlic. Sauté for about 1 minute until fragrant.
Pour in the chicken broth, scraping up any browned bits from the bottom of the pan.
Reduce the heat to medium-low and stir in the heavy cream. Allow it to simmer for about 3-4 minutes until slightly thickened.
Add the grated Parmesan cheese and stir until melted and combined.
Stir in the chopped sage and spinach (if using), allowing the spinach to wilt.
Place the cooked chicken back into the skillet, spooning the creamy sauce over the top. Let it cook for another 2-3 minutes to heat through.
Remove from heat and serve immediately, garnished with extra sage if desired.