Place crock pot insert on stovetop and heat on medium-high. Add 2 tablespoons bacon fat.
Season pork liberally with salt and pepper on all sides, and add to the heated pan. Sear pork about 3-5 minutes per side, until browned.
While roast is browning, cut onion in half, then slice into thin half moons. Repeat with fennel and apple. If using fresh herbs, remove thyme from stalk and roughly chop sage. In a small bowl whisk together bone broth or stock, apple cider, and Dijon mustard. Set aside.
Once the pork is browned, turn off burner and remove pan from the heat. Using pot holders, carefully transfer the pot to the crock pot base. Nestle onions, fennel, and apple around the roast. Sprinkle with herbs and seasonings, then pour stock mixture on top.
Cover with the lid and set timer to 4 hours on low heat. If you're in a rush, cook on high for 2 hours. Once done cooking, remove pork to a plate, cover with foil, and let rest.
Meanwhile, transfer pot back to the stovetop. On medium heat, bring mixture to a boil, then reduce to maintain a simmer. Let cook about 10 minutes, or until sauce reduces to your preferred consistency.
Slice roast into pieces and serve immediately with pan sauce on the side.