• Sea salt and freshly ground black pepper
• Pinch (1/16 tsp.) ground cloves
Peel onions and halve them lengthwise, then cut each half into thirds or quarters. For frittata, slice each piece crosswise about 1/4 inch thick.
Warm olive oil in 10-inch skillet, add onions and cook over medium heat, stirring occasionally until they begin to color. Reduce heat to medium low, cover pan and continue to cook, stirring occasionally, until they've started to color, abut 20 minutes. Remove lid and stir frequently, keeping active eye on them to make sure they don't burn.
When onions are limp and caramel-colored, add vinegar and let it reduce. Season with 1/4 teaspoon salt, lots of freshly ground black pepper and the cloves. Taste, and if you want a little more vinegar, add it carefully. Use immediately or refrigerate for several days. (Can use with pasta, on toast with more vegetables, on grilled meats — or with the frittata recipe.)