Red Onion and Apple Relish

Another "it tastes WAY better then it looks" recipe.  I could eat it by itself but tastes great with a veggie hot dog or on a vegan burger.

1 Tbsp olive oil
1 medium red onion, finely chopped
1/3 cup of sugar
1 Granny Smith Apple
3 Tbsp apple cider vinegar
Salt

In a saucepan, heat the oil over medium heat. Add the onion and stir in the sugar. Cover and cook, stirring occasionally, until the onion is soft, about 20 minutes.
Peel, core and shred the apple and add it to the onion mixture. Site in the vinegar and salt to taste and cook, uncovered, until the mixture is thick, about 20 minutes. Transfer to a bowl and set aside to cool to room temperature until needed, then bring back to room temperature before using. Properly stored, this relish will keep for 2 to 3 days.