Preheat oven to 350.
Melt the duck fat, coconut oil or bacon fat in a large cast iron pan over medium high heat.
Salt and pepper the pork tenderloin on all sides.
Sear the pork on all sides in the hot pan. About 2-3 minutes per side.
Transfer the pan to the oven and cook for 40-50 minutes, or until the pork has reached an internal temperature of 145 degrees.
When the pork has come to temperature, transfer it to a plate and cover loosely with foil to keep warm while you make the sauce.
Place the pan over medium heat.
Carefully pour in the balsamic vinegar to deglaze the pan. Using a rubber spatula, stir the vinegar to loosen up the meaty bits that have stuck to the pan.
Add in the leeks, garlic, and butter. Continue to stir.
Once the butter has melted completely, add in half of the bone broth. Continue to stir until the sauce has reduced by about half.
Add in the remaining broth and stir until the sauce has again reduced by about half.
Pour the sauce over sliced pork tenderloin, and be prepared to lick the plate.
