Heat olive oil over very low temperature. Add the onions and cook and stir until onions become translucent. Cover and simmer until onions are very soft (stir frequently and add a teaspoon of water if it seems as if it will burn).
Once onions are soft, sprinkle the flour, stir vigorously until all the flour has been incorporated into the mix.
Pour in the milk and mix well. Cook stirring until the mix is very thick.
Add the spinach and mix well. Season with salt and pepper and remove from the heat. The spinach will continue cooking in the heated pan.
To assemble
Carve a deep pocket into each chicken breast. Season them with salt, pepper and paprika.
Stuff the chicken breast with the onion and spinach mix, careful not to put too much. You can close the pocket with a toothpick.
Heat the oil over medium-high heat in a large skillet (all 6 chicken breasts should fit).
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 3 minutes each side).
Serve with mashed potatoes, or a salad of shaved carrot, cucumber and apple sprinkled with a mild vinaigrette.