In a heavy-bottomed stock pot over medium-high heat, melt the butter. When it begins to froth reduce the heat to medium and stir in scallions and onion. Fry until fragrant, about 3-5 minutes.
Pour in the chicken stock and potatoes and simmer, covered, about thirty minutes or until potatoes are fork tender.
Stir in lovage and simmer, covered, an additional 5 minutes.
Remove from heat and blend with an immersion blender until smooth.
Season with sea salt and freshly ground pepper. Stir in a spoonful or three of heavy cream and enjoy.