Preheat oven to 190°C.
In a large, ovenproof skillet over medium-high heat, heat 2 tablespoons olive oil.
Cook apple and onion until beginning to soften, 4 minutes, then add garlic and sage and cook 2 minutes more.
Season with salt and pepper, then transfer to a bowl and let cool slightly.
Butterfly pork loin:
Using a sharp knife, make a slit lengthwise down the centre of pork, cutting it in half so it opens like a book (don’t cut it all the way through; it should still be attached at the bottom.)
Season inside of Loin with salt and pepper.
Add bacon and apple filling.
Pull two sides of pork around the filling and tie tightly with kitchen string. (We used 4 pieces of string to tie pork in 4 places and trimmed offends.)
In the same skillet, heat 2 more tablespoons of olive oil over medium-high heat. Add pork and sear for 6 minutes, turning every 1 to 2 minutes, until browned.
Place entire skillet in oven and roast 20 minutes, or until internal temperature reaches 63°C.
Remove from oven and transfer to cutting board; cover with aluminium foil and let sit 10 minutes.
Slice pork and serve.