1. Heat oil in a medium saucepan over medium heat. Cook onion until soft, about 3 minutes. Add potato; cook, stirring occasionally, until almost tender, about 3 minutes. Add cucumbers, and season with pepper; cook until almost tender, about 5 minutes.

2. Add stock, cover, and bring to a boil. Reduce heat; simmer until ingredients are tender, about 12 minutes. Let cool slightly.

3. Working in batches, puree mixture in a blender until smooth. (For safety, remove cap from hole in lid, and cover with a dish towel to prevent spattering.) Transfer mixture to a large bowl; stir in buttermilk, dill, and 3/4 teaspoon salt. Refrigerate until chilled, about 2 hours. Divide soup among 4 glasses, and garnish each with a cucumber spear.