Heat the olive oil in a large sauté pan over medium heat.
Add the sliced onion and cook for 5 to 6 minutes, until softened and translucent.
Add the sliced peppers and a pinch of salt. Cook for 15 to 20 minutes, stirring occasionally, until the peppers are very soft and beginning to caramelize.
Stir in the garlic and cook for about 30 seconds, just until fragrant.
Pour in the tomatoes and stir well to combine.
Reduce the heat to low and simmer uncovered for 30 minutes, stirring occasionally, until the peppers are fully softened and the mixture is thick and saucy. If the mixture becomes too thick while simmering, add a tablespoon or two of water as needed to loosen.
Stir in the red wine vinegar during the last few minutes of cooking. Taste and adjust seasoning with salt and pepper.
Remove from heat. Stir in the torn basil.
Serve warm, at room temperature, or chilled. Pairs beautifully with grilled meats, eggs, or crusty bread.