Place butter in a large Dutch oven. Saute leeks and fennel until browned and soft about 7 minutes. Add wine and deglaze scraping up bits and let reduce by half.
Add chicken broth and bring to a boil. Add potatoes, white pepper, mustard, rosemary, and thyme. Cook until potatoes are tender. Remove from heat and puree with immersion blender or in food processor until smooth. Season with salt. Garnish with fennel fronds in desired.