Squeeze as much water out of thawed spinach as possible. I find this easiest by pushing with the back of a spoon in a sieve. Add to a medium-sized mixing bowl with the rest of the 'filling' ingredients and use a spatula to combine. Check for seasoning then rest in the fridge until needed.
In a small pot combine the 'chicken' seasoning and place to one side.
One by one, place your hand on top of a chicken breast and horizontally slice a pocket into the side. Go as wide and deep as you can, without slicing all the way through. I usually slice on the more rounded side, the side without the tender attached. Season both sides with the seasoning mix.
Open up the chicken breasts and spoon in the filling (you may have a little leftover depending on the size of the breasts). Push it right in, then fold over the pocket to cover the filling.
Wrap each breast in 3-4 slices of bacon, then place into a very large cast iron skillet or baking dish. I usually ensure the bacon ends are underneath the chicken, just so it doesn't unravel in the oven. I also try and cover the gap at the side as much as possible, so the filling doesn't leak out too much.
Bake in the oven at 200C/400F for 25mins, or until the chicken is piping hot & white all the way through, with the filling bubbly and seeping out the sides. You can then flick on the grill and broil for a few mins if you want to crisp up the bacon.