Chop bacon and cook in a skillet to render fat. Set bacon aside and separate rendered fat in a separate bowl.
Add 2 tbsp of bacon fat back to the skillet and cook the sliced shallots over medium heat until soft. Add the garlic and cook gently until fragrant. Do NOT overcook the garlic or it will be bitter.
Add the apple cider vinegar and deglaze the pan, ensuring no bits remain at the bottom of the skillet. Add the brown sugar and Dijon mustard and whisk constantly until incorporated.
Season with salt and pepper and continue whisking. (Be generous with the pepper!).
While constantly whisking, slowly drizzle in 1-2 tbsp of additional bacon fat. Keep whisking so the sauce doesn’t break.
Taste and make any necessary adjustments.
Remove from heat and add the cooked bacon pieces back to the skillet. Stir to coat well.
