Step 1

In a medium pot over medium heat, melt butter. Stir in onions and salt. Cook, stirring occasionally, until onions are translucent and just starting to have some color, about 6 minutes.

Step 2

Add fennel, milk and fennel seed. Increase heat to high and bring to a boil. Reduce heat to low and simmer until fennel is tender, about 10 minutes.

Step 3

Transfer soup to a blender and purée until smooth, about 1 minute. Stir in lemon juice and fish sauce. Season with more salt as needed.