Remove the outer leaves from the radicchio and cut the head in half through the root end. Cut each half into 4 wedges through the root end and remove the white core.
In a small bowl, stir the vinegar and sugar until dissolved.
Pat the scallops dry and season with salt and pepper. Heat a 12-inch skillet (preferably cast iron) over medium heat until hot. Add the pancetta and cook, stirring often, until crisp, about 5 minutes. Remove the pancetta from the skillet with a slotted spoon. Pour off all but 3 Tbs. of the fat (if you have less than 3 Tbs., add olive oil to make up the difference).
Put the skillet over medium-high heat. Add the scallops and sear until golden-brown on one side, 2 to 3 minutes. Flip and continue to cook on the second side until they feel firm when pressed, 2 to 3 minutes more. Transfer to a plate, cover, and keep warm.
Add the onion and rosemary to the skillet and cook, stirring often, until the onion is softened, about 3 minutes. Add the garlic and cook, stirring, until fragrant, 15 seconds. Add the radicchio and pancetta, season with salt and pepper, and cook, tossing gently and occasionally, until the radicchio is wilted, 2 to 3 minutes. Remove the skillet from the heat and pour the vinegar mixture over the radicchio. Toss to combine. Serve with the scallops.