In a large saucepan, saute the leek, carrot and red pepper in oil until crisp-tender. Add garlic; cook 1 minute longer. Stir in stock and potatoes. Bring to a boil. Reduce heat; cover and simmer for 5 minutes. Stir in the asparagus, rhubarb, sugar, salt and pepper; cover and simmer 4-6 minutes longer or until vegetables are tender.