Pat the pork belly dry. Flip the pork over onto its skin and generously season the bottom and edges of the belly with kosher salt. Sprinkle with 2 tsp fennel seeds, rubbing the seasoning into the meat. Flip the belly over and place it skin side up on a rack placed over a baking sheet. Pat the skin dry and refrigerate uncovered overnight to dry out the skin.
Preheat the oven to 450°F. Drizzle 1 tbsp olive oil in the bottom of a roasting pan. Scatter the sliced onion, garlic cloves, and sage sprigs in the roasting pan. Sprinkle with the remaining fennel seeds. Use a sharp knife to score the skin crosswise into about 3/4 inch wide strips, being sure not to cut into the meat.
Place the pork belly, skin side up in the roasting pan. Drizzle the skin with a little olive oil, rubbing it all over. Season the skin generously with kosher salt.
Roast for 30 minutes and then turn the heat down to 325°F. Add the apple juice and chicken broth to the roasting pan, being careful not to pour it over the pork skin. Continue to roast the pork for another 2 to 2 1/2 hours adding the apples after 1 1/2 hours. Roast until the pork is very tender and a skewer easily pierces the meat.
When the pork is tender, turn the temperature of the oven back up to 450°F in order to crisp up the skin. Roast for another 20 minutes until the pork is well browned and the skin begins to puff. Remove the apples from the roasting pan and pour off and reserve the pan juices.
Turn the oven to broil and return the pork to the oven. Watch the pork carefully for 3 to 5 minutes, rotating the pan every couple of minutes. Broil until the skin has bubbled and puffed up. Don't leave the pork unattended as it can easily burn.
Transfer the pork belly to cutting board. Skim the fat from the pan juices – there will be a lot of fat. You should have about 2 cups of pan juices. Add more broth if needed.
Return 3 tablespoons of the fat to the roasting pan, or skillet, and place over medium high heat. Whisk the flour into the fat to make a roux. Let the roux cook briefly. Add the pan juices, half a cup at a time, whisking constantly to prevent lumps. Bring the gravy to a boil and cook until thickened. Adjust thickness if you prefer by adding more broth.
Season the gravy with kosher salt and pepper to taste. Strain the gravy through a fine mesh strainer.
To carve the pork, use a sharp knife to cut the belly into slices between the scored strips of skin. Cut the apples in half to serve. Serve the pork slices with the apples and pan gravy. Enjoy!