Heat the olive oil in a saucepan over medium heat until shimmering and fragrant.
Add the finely diced onion and sauté until translucent, about 5 minutes, until it fills the kitchen with a sweet aroma and becomes soft.
Stir in the minced garlic and cook for another minute until fragrant, making the mixture slightly bubbly.
Add the chopped tomatoes to the saucepan, stirring to combine them with the onion and garlic base.
Pour in the balsamic vinegar, stirring well to coat the tomatoes, and bring the mixture to a gentle simmer.
Reduce the heat to low and let the sauce simmer gently, stirring occasionally, for about 30-40 minutes until the mixture thickens and develops a glossy, caramelized appearance with rich aroma.
Season the sauce with salt and pepper to taste, stirring to ensure even flavor distribution.
Remove the sauce from heat once it has achieved a deep color and a thick, jam-like consistency.
Transfer the sauce to a bowl or serve directly, allowing it to cool slightly before using as a condiment or meal accompaniment.