1 tablespoon vegetable oil (I use grapeseed or sunflower oil)
4 large onions, thinly sliced pole-to-pole (about 8 cups)
Kosher salt and freshly ground black pepper
1/4 cup sugar
2/3 cup balsamic vinegar
1. Heat the oil in a large skillet over medium heat until it slides easily across pan. Add onions, salt, and pepper and cook, stirring occasionally, for about 20 minutes, until onions are soft.
2. Add sugar and reduce the heat to medium-low. Cook, stirring frequently, for about 10 minutes, until onions appear dry.
3. Add vinegar and reduce heat to low. Continue cooking, stirring occasionally, for about 1 hour, until onions are soft and dry. Serve warm or at room temperature. Store marmalade in refrigerator. It will keep for several weeks. Makes about 2 cups.