Trim stem of cauliflower and core it out (keep), trim off about 1 cup of small florets, cut remaining head into ½” slices.

In heavy bottom pot, heat half of the butter and sauté leek and onions until softened. Transfer to a plate and set aside. Add 4 cups of water to pot, half of the sliced cauliflower and core, and bring to a boil. Reduce and simmer for 15 minutes. Add in the remaining cauliflower and simmer for an additional 20 minutes until everything is completely tender. Pour everything into a blender (do in batches if necessary), and process until smooth (leave top hole for heat to vent).

In a small skillet heat remaining butter and cauliflower florets over medium-high heat. Stirring frequently until florets are lightly golden and butter has browned, about 7 minutes. Transfer florets to plate, and reserve browned butter for garnish.

Return pureed soup to pot to heat, and sherry and season with salt and pepper. If too thick, thin with some water.

To serve, garnish each bowl with some browned florets, chives, and a drizzle of the browned butter.