Melt butter in a saucepan over low heat, and cook and stir the potatoes and onion until the onion starts to become translucent, about 3 minutes. Raise heat to medium, pour in the vegetable broth, and bring to a boil. Reduce heat to a simmer, cover, and cook until the potatoes are tender, 10 to 15 minutes. Pour the mixture into a large bowl, and puree through a food mill back into the saucepan. (Discard any fibrous vegetables left in the food mill.)
Bring the pureed soup back to a simmer over medium-low heat, and whisk in 6 tablespoons of mascarpone. Season with salt, pepper, and lemon juice to taste.
Preheat oven to 200 degrees F (95 degrees C), and warm 4 oven-safe soup plates for about 5 minutes.
Roll a slice of smoked salmon loosely into a rose shape, pinching the roll at the bottom and fanning the roll out at the top. Place a rosette of salmon into the bottom of each warmed soup plate, gently pour the soup over the rosette, and drizzle each bowl with about 1/2 tablespoon of mascarpone cheese. Sprinkle with chives, and serve.
Try maybe holding a dill "branch" above the plate and gently give it a hair-cut so the greenery falls on the soup and salmon, using dill instead of the chives. You could also lay two long unchopped chives across the centre of the plate for a simple and nice decoration.