In a Dutch oven over medium heat, melt the butter. Stir in onion. Cover and cook for about 4 minutes, stirring occasionally. Remove the lid and continue cooking for an additional 4-5 minutes. Until the onions are softened.
Pour in the chicken broth, peas, tarragon, rosemary, and salt. Increase the heat to medium-high and bring to a boil. Reduce heat to low, cover, and simmer until the least are tender (about 12-15 minutes).
Use an immersion blender to purée the peas until desired consistency.
Whisk in the sour cream until incorporated.
Salt and pepper to taste.
Garnish with grated Parmesan and fresh parsley.
Serve.