Remove the stalks and, if necessary, any brown or bruised part from the fennel bulbs. Reserve any feathery tops. Cut the bulbs into segments and then into 1 cm (½-inch) cubes.
Put the oil, fennel, onion and garlic in a heavy-based saucepan and fry gently over low heat for 10 minutes, stirring frequently.
Add the tomatoes with their juice, and salt to tas