1. Preheat the oven to 425 degrees. Place the peppers in a roasting pan and roast, turning every 15 minutes, for 45 minutes, until the skins are wrinkled and slightly charred.

2. Transfer the peppers to a cooling rack. When cool enough to handle, peel the peppers over a bowl, allowing the juices to collect in the bowl. Discard the skins.

3. Combine the peppers, collected juices, and balsamic vinegar in a food processor and process until smooth. Season with salt.