There are two drawbacks to cooking on the fly:
1. You don't tend to measure so writing things down after is an effort in guess work
2. You always 2nd guess a decision and come up with better ways after the fact
With those things in mind I'm going to write the recipe as I think it should be done rather than the slapdash, albeit delicious, first attempt.
3-4 lb pork loin roast (not pork tenderloins please)
1 lb bacon
1 cup caramelized onions
1/2 cup shitake mushrooms (dried is fine about 8-10 dried)
1 cup cooked spinach
1/2 cup shredded cheddar cheese
4 feet butchers twine
Preheat your oven to 350
Slice the pork loin horizontally, as close to the center as you can get, do not slice it all the way through. Open it up so it lays flat. Let it rest while you prep the rest of your ingredients.
Re-hydrate your mushrooms if you are using dried. I like to use a mix of boiling water and wine to re-hydrate Just plain water will work but I find the wine gives them a better flavor. Let them steep until soft then chop finely. If using fresh just chop them up. I don't like shitake stems as I find them too woody in texture so remove them.
If you are using fresh spinach, saute it with a bit of chicken stock and mix in your chopped mushrooms. Salt and pepper to taste. For frozen spinach thaw in the microwave and saute with the mushrooms to heat the through.
Once your spinach and mushroom mix is cooked, spread it in an even layer inside of your butterflied pork, then layer on the cheese and onions.
Close the pork and truss it with your butchers twine.
Once you have your pork all tied up and lovely, flip it over so most of your knot work is on the bottom then drape it with bacon. You can skip the bacon and brown it before baking but why would you skip the bacon?
Stick it in the oven and cook until it reaches an internal temp of 145-150. Yes I mean it, don't over cook your pork. The USDA says its ok. Let it rest for at least 5-10 minutes after it reaches temp.
Once it is done you should be able to roll it onto its side and cut the twine from the bottom. By doing so you can pull it off without disturbing your nice crispy bacon.
Slice into pork chop thickness pieces and serve.