And peeled potatoes to a large pot. Add enough water to cover and boil until tender.
In a stock pot over medium high heat, add all ingredients for beef. Cook and crumble until no longer pink. Remove beef from pot.
Add 4 tbsp butter, onion and garlic to same pot and reduce heat to medium. Cook for 3 minutes. Then add broth, tarragon, parsley, red pepper flakes, and mustard seeds. Let come just to boil.
Drain potatoes in colander. Transfer to bowl and add sour cream, heavy cream and remaining butter. Smash until mostly combined.
Transfer potatoes to stock pot. Use an immersion blender to blitz until smooth. Add cooked beef, peas, carrot, corn, and white pepper. Stir to combine. Let simmer for 15 minutes.