Potato and Onion Cake

http://www.marthastewart.com/334291/potato-n-onion-cakes
Yield Makes 6

Ingredients

2 tablespoons unsalted butter, softened, plus more for tin
Coarse salt and freshly ground pepper
1 tablespoon packed dark-brown sugar
1 tablespoon balsamic vinegar
1 tablespoon red wine vinegar
2 teaspoons finely chopped fresh rosemary, plus 6 sprigs for tin
1 small red onion, sliced into six 1/4-inch-thick rounds
4 medium Yukon gold potatoes, coarsely grated
1 large egg yolk, lightly beaten

Directions

Preheat oven to 400 degrees. Generously butter a large (six 1-cup) muffin tin. Sprinkle each cup with salt, pepper, and 1/2 teaspoon sugar; drizzle each cup with 1/2 teaspoon of each vinegar. Put a rosemary sprig in each cup, and cover with an onion round; set aside.
Toss potatoes with chopped rosemary and egg yolk in a medium bowl; season with pepper and 2 teaspoons salt. Divide among cups; dot with butter.
Bake until potatoes are tender and well browned, about 30 minutes. Remove from oven. Let cool 5 minutes. Run a thin knife around edge of each cup to loosen; invert, and serve.