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.