In a large nonreactive saucepan, heat the oil. Add the fennel and onion; cook over moderate heat, stirring, until the onion is translucent and beginning to brown, about 5 minutes. Add the garlic; cook, stirring, until fragrant, about 1 minute. Blend in the tomato paste. Add the stock and the tomatoes with their juices. Season with salt and pepper; bring to a simmer. Reduce the heat, cover and simmer gently for 20 minutes.