In a large saucepan or Dutch oven,
saute onion, leeks and garlic in olive oil for about 5 minutes over medium-high heat. Add beans, potatoes, 1/4 tsp thyme and 1 1/2 cups vegetable broth; simmer 20 minutes. Puree in blender or food processor. When smooth, return to saucepan and add remaining broth and evaporated milk; heat through. If necessary, add more water or broth to reach desired thickness.
Before serving, stir in chopped parsley, lemon juice, salt and white pepper ans remaining 1/4tsp thyme.