Plastic-wrap the Fatty Ground Pork (14 ounce) and flash-freeze for 2 hours. This is important if you want to create that "bouncy" texture of sausages.
Combine All-Purpose Flour (3 cup), aluminum-free Baking Powder (1 1/2 tablespoon), Baking Soda (3/8 teaspoon) and Salt (1/3 teaspoon) in a large bowl (or food processor), whisk (or pulse) until mixed evenly.
Add the Unsalted Butter (8 1/2 tablespoon) with a pastry-cutter (or pulsing in a food processor), until the largest bits are the size of peas. Transfer the mixture to a large bowl and with your hands, squeeze the butter into flat disks.
Add Buttermilk (1 cup) and fold the mixture gently together until it forms a dough, which will seem slightly on the dry side.
Gently roll the dough into 1/2-inch (1.5 cm) thickness.
Cut the sheet into 4 slabs, then stack them on top of each other (this is what's gonna create the layers), and roll again into 3/4-inch (2 cm) thickness.
Cut out the as many biscuits you can with a round-cutter, gather the scraps, and cut again. You should have about 9~10 biscuits. Place the biscuits on a parchment-lined plate, and freeze for 2 hour until frozen. Can be made the day ahead.
Peel the Fuji Apple (1) through the smallest holes on a cheese grater. Squeeze out all the excess juice, place in a food-processor with Shallot (1) and Garlic (1 clove), and run until finely minced.
Add the cold fatty ground pork, Corn Starch (1 tablespoon), Salt (2 teaspoon), Maple Syrup (2 teaspoon), Fennel Seeds (1 teaspoon), Ground Black Pepper (1/2 teaspoon) and Ground White Pepper (1/4 teaspoon). Run food processor until evenly incorporated.
Keep it running while you slowly add in Water (4 tablespoon), and stop until the mixture is smoothly blended. You may be able to do this by hand by whisking vigorously.
Heat a large, oven-proof skillet over high heat. Cook the sausage with a bit of vegetable oil, breaking it into small pieces with a wooden spoon, until nicely browned.
Leave 1 tbsp of dripping in the skillet, then transfer the sausages to a bowl and set aside. Lower the heat to medium then add Unsalted Butter (2 tablespoon) to the skillet.
Cook the Onion (1) and a pinch of salt until very soft and translucent, approx 10 minutes. Add the All-Purpose Flour (3 tablespoon) and cook for a couple minutes.
Add the Milk (3 cup) and Chicken Stock (1/4 cup). Keep whisking until the mixture has come to a simmer and starts to thicken. Add the browned sausage back to the skillet.
Season with Ground Black Pepper (1 tablespoon), Ground White Pepper (1 1/4 teaspoon), Worcestershire Sauce (1/2 teaspoon), Cayenne Pepper (1/2 teaspoon) and Ground Nutmeg (1/4 teaspoon). Simmer for 6 to 8 minutes. Re-season with salt if needed.
Preheat the oven on 500 degrees F (260 degrees C). Bring the gravy back to a simmer if needed (if the gravy seems too thick at this point, loosen with a bit more chicken stock), then mix in the Dijon Mustard (1 1/2 tablespoon).
Arrange the frozen biscuits over the gravy, and brush the top with egg wash. Bake in the oven for 10 min.
Then lower the heat down to 430 degrees F (220 degrees C), and bake for another 15 min until the biscuits are puffed and golden browned. If the biscuits are browning too fast before the end of 15 min, lower the heat down to 400 degrees F (200 degrees C).
Serve the casserole immediately with chili flakes and tabasco sauce.