Season the chicken with the salt & pepper, making sure to mix it in evenly.
Heat oil in a skillet over medium-low heat.
Place the chicken thighs skin side down in the skillet and cook for about 13 minutes in order to crisp up the skin. Move the chicken around from time to time to ensure it cooks evenly. The skin should be hard to the touch.
Flip the chicken, then cook for about another 15 minutes until the chicken is fully cooked. Remove from pan.
Add the garlic and onion. Stir in the tomatoes and spinach until the spinach is wilted.
Add the heavy cream, salt, and pepper, bringing to a boil.
Add the parmesan cheese, stirring until the sauce has thickened slightly.
Place the chicken back in the pan, spooning the sauce on top of the chicken.