Peel and chop the onion as well as the garlic cloves. Divide the chopped onion and garlic equally into two parts.
Add 1 tbsp olive oil to a pan over medium heat, and add one of the parts of onion and garlic to it and sauté it for about 5 minutes.
Add the frozen spinach and turn the heat up a bit so the liquid in the spinach evaporates out.
Sauté the spinach with onions and garlic for about 5-7 minutes, until the spinach is heated through. Season with a pinch of salt and pepper. Take the mixture off the heat and set aside to reach room temperature.
Meanwhile, lay out the 4 chicken breasts and pound them out until they are flattened. Season with salt and pepper.
Then, add a layer of the spinach mixture on each and top it with a little bit of the feta. Do this with all the chicken breasts.
Then, gently roll the chicken breasts up and secure with a toothpick.
Using the same pan that the spinach was sautéed in, add the remaining tbsp of olive oil to the pan, place it over a stovetop set to medium heat, and place the rolled and secured chicken breasts into the pan.
Let the chicken sauté for about 5 minutes or until browned.
Flip the rolled, stuffed chicken over so the browned side is facing up.
Add in the remaining parts of the onions and garlic and the tomato sauce. Cover and let simmer for 15 minutes.
When time is up, remove the pan from the heat. Spoon a bit of the sauce over the chicken. Sprinkle on some parsley and more feta.
Serve as is or with a side of veggies, salad, or bread.