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

Ingredients

• 4 tablespoons unsalted butter, softened, plus more for tin
• kosher salt and freshly ground pepper as needed
• 2 tablespoons packed brown sugar
• 2 tablespoons balsamic vinegar
• 2 tablespoons 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