Step 1To roast the peppers, stand them on a cutting board and cut the sides off in flat slabs. Arrange the slabs, skin side up, on a foil-lined baking sheet. Broil them about 6 inches from the heat until the skin is blackened, about 10 minutes. Wrap the peppers in foil until cool. Slip off the skins and cut the peppers crosswise into strips.
Step 2Heat 2 tablespoons of the oil in a large nonstick skillet over medium-high heat. Add the onion, salt and peppers. Cook, stirring often, until the onion is very soft but not browned, about 8 minutes.
Step 3Add the garlic and 1 cup of vegetable broth and bring it to a boil. Reduce the heat and simmer, uncovered, stirring often and adding more broth as needed to keep the mixture moist but without making it soupy. Cook until the mixture thickens, about 15 minutes.
Step 4Add the raisins, sugar, capers, vinegar and a dash of red pepper flakes. Raise the heat to high and cook, stirring, until the mixture begins to brown, about 2 minutes. Stir in the remaining 1 tablespoon of oil. Taste and adjust the seasoning. Cool and refrigerate overnight or up to 3 days.
Step 5To serve, let the mixture come to room temperature; stir well and taste and adjust the seasoning, including sugar and vinegar. Spoon onto the crusty bread to serve.