In a 4- to 5-quart pan over medium-high heat, melt butter. Add onion and stir often until golden, 7 to 10 minutes.
Add flour and stir to coat onion. Add broth, artichoke hearts, and tarragon; stir until mixture boils and thickens, 15 to 20 minutes.
In a blender, holding lid down with a towel, whirl mixture in batches until smooth; pour into a large bowl.
Return soup to pan and stir over low heat until hot. Ladle into bowls and garnish with spoonfuls of whipped cream and Parsley-Lemon Gremolata. Add salt and pepper to taste.