Heat oil in a large Dutch oven over medium heat. Add onion and garlic to pan; cook 8 minutes or until onion is tender, stirring occasionally.
Add chopped cucumber, potato, and 2 cans broth to pan; bring to a boil. Cover, reduce heat, and simmer 20 minutes or until potato is very tender, stirring occasionally.
Place one-third of potato mixture in a blender. Remove center piece of blender lid (to allow steam to escape); secure blender lid on blender. Place a clean towel over opening in blender lid (to avoid splatters). Blend until smooth. Pour into a large bowl. Repeat procedure twice with remaining potato mixture. Cool.
Add remaining 1 can chicken broth, buttermilk, juice, salt, and pepper to cooled potato mixture, stirring well. Cover and chill. Garnish with thinly sliced cucumber.
MyRecipes is working with Let's Move!, the Partnership for a Healthier America, and USDA's MyPlate to give anyone looking for healthier options access to a trove of recipes that will help them create healthy, tasty plates. For more information about creating a healthy plate, visit www.choosemyplate.gov.