Add remaining 1 tablespoon oil to the pot. Add onion, carrots and chopped fennel bulb; cook over medium-high heat, stirring frequently, until the vegetables are softened and beginning to brown, 3 to 4 minutes. Add garlic and crushed red pepper; cook until fragrant, about 1 minute. Add broth, potatoes and salt; bring to a boil over high heat. Reduce heat to medium, return the chicken to pot, partially cover and cook until the potatoes are tender and the soup has thickened slightly, about 35 minutes. Remove from heat; stir in chopped fennel fronds and sour cream. Ladle into bowls and sprinkle with more fennel fronds, if desired.