Spray a large saucepan with oil and cook leek over medium heat for 4 minutes, until softened. Spray pan with a little more oil, add garlic and cook for 1 minute. Add tomatoes and 4 cups water. Bring to the boil, reduce heat and simmer, uncovered, for 10 minutes.
Add beans and silverbeet to pan and cook for 5 minutes, or until silverbeet wilts. Top soup with sour cream and serve with cheesy bread.