Spike Mendelsohn's Red onion marmalade

To make the red onion marmalade, slice both red onions 1/2 inch thick. Add the vinegar and sugar to a pot over medium heat. Bring to a simmer. Once the sugar is completely dissolved, add the onions. Cook, stirring constantly to prevent burning, until the onions are translucent and the liquid is reduced by half, about 5 minutes. Set aside to cool.