Preheat the oven to 400 F/ 200 C/ gas mark 6. Line a medium size oven tray with parchment paper and set aside.Place a chicken breast on top of a board and cut a pocket into the thicker side of the chicken being careful not to cut through the opposite side. Repeat with the other chicken breasts and place them on top of the baking sheet. Season lightly on top and inside. Set aside.
Place the spinach in a food processor and pulse a few times until finely chopped (but not mushy).
In a mixing bowl combine the spinach with the ricotta, 4 tablespoons of the breadcrumbs, grated garlic, onion granules, salt and pepper to taste and stir thoroughly.
Stuff the chicken breasts with the mixture flattening a little to remove any air.
In a small bowl combine the remaining 6 tablespoons of panko with the parmesan and stir thoroughly. Coat the tops of the chicken breasts with oil (do this thoroughly) then cover with the crumbly mixture pressing down gently so it sticks to the chicken. Spray the tops lightly with cooking spray.
Bake in the centre of the oven for 20-23 minutes. Remove from the oven and serve immediately.
You can either pulse the spinach briefly in the food processor or finely chop it.
Ensure your spinach is dry before pulsing. Pat it dry using paper towel if needed.
I recommend using baby spinach as it's tastier than ordinary spinach.
I do not recommend using fine breadcrumbs instead of panko.
Season the chicken as well as the filling.
The exact cooking time will depend on the size of your chicken. The chicken is ready when the juices are clear. Try not to overcook it.