Gather all ingredients before starting the recipe.
Peel 3 medium red onions, cut in half, then slice into 1/2" thick slices.
Heat 2 tbsp olive oil in a medium sauce pan with a lid over medium high heat. Add sliced onions and season with kosher salt and black pepper.
Cook on medium heat, with the lid on for about 25-30 minutes, or until the onions are soft.
Add 1/2 cup red wine vinegar and 1/4 cup sugar, stir to combine, reduce heat to medium low, and simmer the onions until syrupy and jam-like. Let cool.
Spoon the jam into a resealable jar and store in the fridge.
