Preheat oven to 400°.
Reduce vinegar and sugar over medium heat until slightly thickened and reserve. In a small pan add 8 tablespoons butter (1 stick) and brown over medium heat. Set aside in a warm place.
Meanwhile cook parsnips in boiling water until tender, drain, reserving 1 cup cooking liquid. Add parsnips, cream, ½ cup cooking liquid and 4 tablespoons butter to bowl of food processor.
Pulse machine until smooth, adding additional cooking liquid until desired consistency is reached. Season to taste. Keep warm. (Can be made 1 day in advance and reheated.)
Heat olive oil in a large nonstick, ovenproof skillet. Add Brussels sprouts, cut side down, sprinkle with salt and pepper to taste and cook over medium high heat until cut sides are light brown.Transfer to oven and cook until just tender, 5–7 minutes. Remove from skillet and keep warm.
Heat vegetable oil in same skillet over medium-high heat until shimmering. Sprinkle scallops with rosemary salt, place in pan to sear until nicely caramelized, 1–2 minutes per side depending on size. (Do not crowd pan, searing in two batches if necessary.)
Divide purée and Brussels sprouts between 6 plates, top with 4 scallops each and drizzle with butter and balsamic glaze.
Recipe by Wayne Bush, Matunuck Oyster Bar