Cook bacon in heavy Dutch oven over medium heat until golden brown, stirring occasionally, about 10 minutes. Transfer to paper towels using slotted spoon.
If necessary, add enough oil to pan drippings to measure ¼ cup. Heat over medium heat.
Add onion and cook 4 minutes, stirring frequently.
Add fennel and cook 5 minutes, stirring frequently.
Add stock. Simmer until vegetables are tender, 35 - 40 minutes.
Puree soup in batches in processor or blender until smooth. (can be prepared 1 day ahead and refrigerated.)
Reheat soup over medium heat, stirring frequently.
Whisk in one cup creme fraiche. Season with salt and pepper.
Ladle soup into heated bowls. Spoon 1 ½ Tbsp creme fraiche atop each. Garnish with chives and bacon.