1 tablespoon unsalted butter1 tablespoon extra-virgin olive oil2 medium onions, thinly slicedSalt2 baking potatoes (1 3/4 pounds), peeled and sliced crosswise 1/2 inch thick1/4 cup rendered duck fat, or or melted unsalted butter1 large garlic clove, choppedFreshly ground black pepperPinch of crushed red pepper2 teaspoons fresh lemon juice2 tablespoons chopped parsley
In a medium saucepan, melt the butter in the olive oil. Add the sliced onions and a large pinch of salt. Cover and cook over moderate heat, stirring occasionally, until the onions are very soft and golden, about 20 minutes.
2.Meanwhile, put the potato slices in a large saucepan of water, add a large pinch of salt and bring to a boil. Simmer over moderately high heat until the potatoes are just tender, about 5 minutes. Drain the potatoes and spread the slices in a single layer on a baking sheet; let cool to room temperature. Gently pat the potato slices dry.