Deep-red beets offer beautiful contrast to the bright-green chilled spinach-pea soup hors d'oeuvre and orange butternut-squash soup hors d'oeuvre. If you prefer yellow soup, substitute golden beets for the red beets in this recipe.
Heat the oil in a large pot over medium heat. Add onion, season with salt, and cook, stirring occasionally, until softened, 5 to 7 minutes.
Add beets, stock, and enough water to cover (4 to 5 cups) to the onion. Bring to a boil, then reduce heat to medium, and simmer until beets are tender and can be pierced easily with the tip of a paring knife, 20 to 25 minutes.
Puree stock and vegetables in a blender until smooth. (You may need to work in batches.) Transfer pureed soup to a clean pot, and adjust consistency with water if necessary (you should be able to drink soup easily without using a spoon). Season with salt, pepper, and lemon juice. Use a glass measure to pour soup into 2-ounce sake or espresso cups, and serve warm.