1 large red onion
5 tablespoons red wine vinegar
1 tablespoon sugar, or to taste
1/4 cup water, plus more as needed
Extra-virgin olive oil
Combine the vinegar, sugar to taste, a pinch of salt and the water in a liquid measuring cup; stir to dissolve the sugar.
Combine a little of the oil and all the sliced onion in a large skillet; heat over medium heat, stirring to coat the onion evenly. Reduce the heat to medium-low; cover and cook for about 10 minutes, stirring as needed, until the onion is soft.
Add the vinegar mixture; cook uncovered for 5 minutes, until the liquid has evaporated and the onion begins to caramelize. Transfer to a bowl and cover loosely to keep warm.
Serve over seared fish or meat.