This recipe is awesome as is, even if you’re not vegan. But if you want, you can substitute the same amount of unsalted butter for the olive oil (for the non-vegan version).
In a large, heavy-bottomed sauté pan, heat the olive oil over medium-low heat. Add the onions and salt. Cook, stirring every few minutes, for at least 1 hour, until the onions are very deep in color. Keep in mind that this process is very slow and could take up to 2 hours.
When the onions are almost translucent and browned, add the vinegar. Taste and adjust the