In a 4 quart saucepan, heat the olive oil over medium heat. Add sliced onions and salt, stir to combine. Cook, stirring occasionally for 10 minutes or until soft and lightly golden. (Keep an eye to make sure the onions don’t burn)
Add tomatoes and sugar, cook 5 minutes or until tomatoes start to break down into a sauce. Using a spatula, smash the tomatoes down into a sauce. Cook 1 -2 more minutes.
Add balsamic vinegar, reduce heat to medium-low and cook for 10 minutes, stirring occasionally until the jam comes together.
Cool 10 minutes before serving.
Store covered in the refrigerator for up to 7 days.
Use as a flavorful sauce for burgers, sliders and sandwiches.