Using a meat mallet, flatten the halved chicken breasts between 2 sheets of parchment to a uniform chicness and season each side with salt and pepper. You can do this step in advance and reserve the chicken in the fridge.
Heat 1 tablespoon of olive oil and the butter over medium heat in a sauté pan. When hot, add the minced onion and cook until translucent. Add the garlic and sauté for 2 minutes. Season with salt and pepper.
Remove from the stove and cool for approximately 5 minutes. Add the egg to the spinach mixture as well as the breadcrumbs.
Take a chicken cutlet and add a teaspoon of filling to the centre. Roll the cutlet and hold in place with 1-2 toothpicks. (Tip: make sure they are not flavoured or you may end up with minty chicken!) Continue with the rest of the chicken.
Dip the rolled cutlets first in the seasoned flour, then the egg and lastly into the breadbrumbs.
Fry in the vegetable oil over moderate heat. Once the chicken is cooked, the joint will seal and you can remove the toothpicks. You do not want to brown the chicken too quickly so that it will have time to cook through.
You can serve the dish with good quality marinara sauce. Enjoy!