If you have a digital thermometer that you can leave in the meat while it's roasting, put it into the meat with the tip centered in the thickest part. Place it all on a rack, and roast at 225' until the thermometer alarm goes off. The meat should be fat side up. Make sure to season all sides of your pork belly before roasting. Score the fat cap with diagonal slices, about an inch apart, taking care not to slice into the meat. Make sure to set the alarm on your thermometer for your target temp.
When the temp is reached, turn off the oven and turn on the broiler. broil the roast until the fat cap is crispy brown. Use any pan juices to make a sauce, or gravy.
My goto side for this would be chunky smashed spus with the gravy, or butter, and a side of a wet cole slaw.
Hope this helps.
Seeeeya; Chief Longwind of the North