Melt butter in a saucepan over medium heat and cook onion until soft and translucent. Whisk in flour and stir until a paste forms, 3 to 5 minutes. Whisk water and vegetable bouillon into the saucepan. Add fennel and bring to a boil, reduce heat, and simmer until fennel is soft, 15 to 20 minutes.
Puree soup with an immersion blender. Stir in cream. If soup is too thick, add more water as needed. Season with salt and pepper.
Chop fennel fronds. Divide soup between 4 bowls and sprinkle with fennel fronds.
