In a large saucepan, melt the butter over medium-high heat. Add the onions and cook until soft, about 3 minutes. Add the garlic, and cook, stirring, for 30 seconds. Add the potatoes, marjoram, salt, and pepper, and cook, stirring, for 1 minute. Add the stock and bring to a boil. Reduce the heat to medium-low and simmer, stirring occasionally, for 15 minutes. Add the watercress and cook until the potatoes are tender, about 10 minutes.
Remove from the heat and puree with a hand-held immersion blender, or in batches in a food processor or blender. Add the cream and return to medium heat. Cook, stirring occasionally, until heated through, adding more cream, if a thinner soup is desired. Adjust seasoning, to taste.
Drape each potato pancake wedge with a slice of smoked salmon, topped with 2 teaspoons of sour cream, and 1/2 teaspoon of salmon caviar. Serve with soup.
Yield: 6 servings