In a 1 gallon saucepan, heat the oil. When the oil is hot, add the onions. Season with salt and pepper. Saute for 2 minutes. Add the garlic and cucumbers. Season with salt and pepper. Saute for 4 to 6 minutes, or until wilted. Add the stock and bring to a boil. Reduce the heat to a simmer and cook for 30 minutes. Remove from the heat, using a hand-held blender or food processor, puree until smooth. Chill the soup completely. Stir in the yogurt. Season with salt and pepper. Refrigerate until ready to use. In a small mixing bowl, whisk the olive oil and lemon juice together. Whisk in the shallots. Season with salt and black pepper. To serve, ladle the soup into eight bowls. Arrange three of the oysters around each bowl of the soup. Drizzle the sauce over each oyster. Garnish each oyster with the caviar. Garnish the soup with parsley.