Add a medium saucepan over medium heat. Add sliced red onions, red wine vinegar, red wine, sugar, salt, and black pepper.

Once everything is combined and simmering, reduce heat to low and simmer until the liquid is reduced by half. This will take approximately 30 minutes. The goal is for the onions to be soft and the liquid to become syrupy and thickened.

After the onion jam has thickened, stir in lemon juice and season with more salt and pepper, if needed.

Allow to cool fully and store in a sealed container in the refrigerator for up to two weeks.