Onion-Topped Potato & Rutabaga Cakes (Neeps and Tattie Cakes) - SippitySup

Ingredients

• 4 tablespoon unsalted butter, softened, plus more for tin
• kosher salt and freshly ground pepper as needed
• 2 tablespoon packed brown sugar
• 2 tablespoon balsamic vinegar
• 2 tablespoon red wine vinegar
• 1 tablespoon finely chopped fresh rosemary, plus 12 sprig tips for tin
• 2 small red onions, peeled & cut into 12 ¼‑in thick rounds
• 1 pound klondike goldust idaho potatoes, coarsely grated
• 1 pound rutabaga, coarsely grated
• 2 large egg yolks, lightly beaten