Heat olive oil in a large nonstick skillet over medium heat. Add onion, and cook until it begins to soften, 3 to 4 minutes. Add bell peppers, sugar, thyme, rosemary, salt, and black pepper. Toss until well combined. Cover pan, and reduce heat to low. Cook, stirring occasionally, until peppers are very soft, about 30 minutes.
Slice baguettes in half lengthwise close to the bottom. Scoop out some of the top half so the bread will sit nicely on top of the peppers; set loaves aside, and reserve scooped-out bread for another use. Uncover; raise heat to medium. Cook, stirring often, until most of the liquid evaporates and peppers are nicely glazed, about 3 minutes. Stir in balsamic vinegar. Add a little water if necessary, to avoid scorching. Transfer peperonata to a bowl to cool. When cool, peperonata can be stored, refrigerated, up to 4 days.
Spread the herbed goat cheese on bread, and top with cooled peperonata.