Ingredients
2 tablespoons unsalted butter, room temperature, plus more for tin
Coarse salt and freshly ground pepper
3 teaspoons packed dark-brown sugar
3 teaspoons balsamic vinegar
3 teaspoons red-wine vinegar
1 1/2 teaspoons finely chopped fresh rosemary, plus tops of 6 sprigs
1 small red onion, sliced into six 1/4-inch thick rounds
2 medium Yukon Gold potatoes (about 1 pound), scrubbed
1 large egg yolk, lightly beaten
Preparation
Preheat oven to 400 degrees. Generously butter six cups of a large (6-cup capacity) muffin tin. Sprinkle each cup with salt, pepper, and 1/2 teaspoon sugar; drizzle with 1/2 teaspoon each balsamic and red-wine vinegar. Add a rosemary top to each cup; cover with an onion slice. Set aside.
Using a mandoline, slice potatoes into matchsticks. In a medium bowl, toss potatoes with chopped rosemary and egg yolk; season with salt and pepper. Divide among cups; dot each with butter.
Bake until potatoes are tender and edges are browned, about 30 minutes. Run a thin knife around inside of cups to loosen; invert cakes, and serve.
