To prepare the stuffing, heat the butter in a skillet over medium heat. Add the onions and cook and stirring them until they become translucent. Cover with a tight-fitting lid, and simmer until the onions are very soft (stir frequently and add a teaspoon of water if it seems as if it might burn).
Step 2: Finish filling
Once the onions are soft, sprinkle the flour, and stir vigorously until all the flour has been incorporated into the mix and is no longer white. Pour in the milk and stir vigorously to combine.Cook stirring until the mix is very thick. Once the sauce has thickened, add the spinach and cream cheese and mix well. Season with salt and pepper and remove from the heat. The spinach will continue cooking in the heated pan.
Step 3: Stuff chicken
To stuff the chicken, butterfly the chicken breasts with a sharp knife. Season on all sides with salt, pepper, onion powder, garlic powder, and paprika.Stuff the chicken breasts with equal amounts of the cream cheese mixture. Fold, and close the pocket with a toothpick.
Step 4: Sear chicken
To cook the stuffed chicken breasts, heat the vegetable oil over medium-high heat in a large skillet (all 6 chicken breasts should fit in it).Carefully place the chicken breasts on the skillet making sure the stuffing doesn't pour out. Cook the chicken breast until it turns golden brown on one side. Turn and repeat (about 4 minutes on each side).
Step 5: Finish cooking
Lower the heat to medium, and cover with a tight-fitting lid, cooking for two more minutes on each side.Remove the chicken from the stove, remove the toothpicks, and serve.
Cook's notes
If you don't mind the extra fat, use heavy cream instead of milk, it makes for a richer filling.When cooking chicken, please remember good hygienic practices.