Preheat oven to 400°F. Line a baking sheet with parchment paper.
Season the pork belly with salt and pepper, then place it on the prepared baking sheet.
Roast the pork belly in the oven for 35-40 minutes, or until the skin is crispy and the internal temperature reaches 145°F.
While the pork is cooking, make the spiced plum sauce. In a saucepan, combine the chopped plums, brown sugar, apple cider vinegar, cinnamon, ginger, allspice, and nutmeg. Cook over medium heat for 10-15 minutes, or until the plums have broken down and the sauce has thickened slightly.
In a small bowl, whisk together the cornstarch and water to make a slurry. Stir the slurry into the plum sauce and cook for 2-3 minutes, or until the sauce has thickened further.
Slice the roasted pork belly and serve it with the spiced plum sauce on the side. Enjoy!