Pound chicken breast to about 1/2 inch thick. Season with kosher salt. Preheat skillet on medium high heat.
Cook chicken with the olive oil for 5-7 minutes on each side until cooked through or reaches an internal temperature of 165°.
Meanwhile, bring a medium pot of salted water to boil. Cook gnocchi according to the package. Usually about 2-3 minutes or until gnocchi float. Remove gnocchi from pot and set aside. Reserve 1-2 cups pasta water.
Remove chicken from skillet and set aside. In the same skillet, add butter, onion and garlic. Stir frequently, reducing heat if necessary to keep from burning, until onions are translucent and fragrant. About 3 minutes.
Add one cup of pasta water and gnocchi to skillet. Saute for 1-2 minutes more, increasing heat if necessary, until about half of the water has been reabsorbed.
Add parmesan and pepper and stir until combined. 30 seconds.
Add spinach, one large handful at a time, until wilted. Stir frequently.
Remove skillet from heat and stir in cream. Adding an additional tablespoon of pasta water at a time if needed to achieve desired consistency of sauce. I prefer the sauce to be thick where it coats the back of a spoon. Taste and add salt if necessary.
Add chicken back to skillet and serve. Alternatively, you can divide the gnocchi and spinach sauce onto serving dishes and cut chicken in half and place on top and serve hot. Add optional red pepper flakes if desired.