Marcella Hazan says: "An Italian vegetable soup is an excellent illustration of the principle of insaporire, the extraction and building up of flavor. Note how the rapini is sauteed at length with onion that has already been cooked to a golden color. Only then is the red pepper added, after the rapini has been given an opportunity to release and concentrate its flavor. Note too that the red pepper is peeled, so that there is no skin to mask its sweetness; moreover, it is skinned raw, not charred first. Unlike charred bell peppers, the flesh is dry and compact and takes the sauteing more deeply. Although Idaho potatoes are not a variety found in Italy, they are an excellent choice for this soup."