Preheat the oven to 250 degrees F.
For the sausage: Mix the ground pork and back fat together in a mixer. Saute the garlic and chili flakes in a pan until fragrant, and then pour in the wine to deglaze. Let cool, and then mix into the pork mixture with the fennel powder, rosemary and thyme.
Remove the pork belly from the brine and place fat-side up on a work surface. Sprinkle with salt and pepper, and then evenly spread on the sausage mixture. Tightly roll the belly starting at one end. Tie up the roll with twine, spacing 1 inch apart. Roll in aluminum foil and place in a roasting pan. Roast for 8 hours.
Put the potatoes in a pot and cover with water. Add salt and cook over medium heat until soft. Bring cream and butter to a simmer in a saucepan. Once the potatoes are cooked, drain and pass the potatoes through a food mill. Whisk the warm cream and butter into the potatoes. Add the truffle oil and more salt to taste.
Bring a pot of water to a boil and add salt. Blanch the carrots, parsnips and turnips for 3 minutes, and then cool in a water bath. Dry the vegetables dry, and then saute with butter. Add the maple syrup, thyme and salt to taste.
Serve the porchetta with the potatoes and vegetables.