Prep the vegetables: Cut the tops off the bell peppers and slice down one side to open them up. Remove the seeds, then slice into thin strips. Peel the onion, cut it in half, and slice it into thin strips. Thinly slice the garlic.
Sauté the onion and garlic: Heat the olive oil in a wide pan over low to medium heat. Add the onion with a pinch of salt and cook slowly until soft and lightly golden, about 8 to 10 minutes. Stir in the garlic and cook for 1 minute.
Sauté the peppers: Add the sliced peppers and season with another pinch of salt. Stir to coat in the oil, then cover and cook for 10 minutes, stirring occasionally, until they begin to soften.
Simmer with tomatoes: Add the crushed tomatoes and white wine vinegar. Stir to combine. Reduce the heat to low and let simmer uncovered for 30 to 40 minutes. Stir occasionally and taste partway through. Add more salt if needed. The peperonata is ready when the peppers are very soft, and the mixture has reduced to a thick, jammy consistency. Do not rush this step.
Finish with herbs: Taste and adjust with salt and pepper. If needed, add a small additional splash of white wine vinegar or a pinch of sugar to balance the acidity. Stir in fresh herbs just before serving.
Serve: Serve warm or at room temperature. Peperonata tastes even better after it sits for a few hours or overnight.
Notes
If the tomato sauce is too thick, or thicker than you would like, add 1/2 cup of water and stir.
You can substitute white wine vinegar with red wine vinegar.