Add salt, onion, and leek. Cook until onion becomes soft and translucent.
Add half of cauliflower, 4 tablespoons butter, 2 cups broth and 2 cups water to pot. Bring to simmer for 10-15 minutes, until cauliflower is tender and falling apart.
Add ¾ of remaining cauliflower to pot and continue to simmer.
Melt 2 tablespoons butter with thyme and remaining cauliflower florets. Stir as the butter begins to bubble and brown.
Once the cauliflower in the pot is tender, remove from heat and place in blender. Add parmesan cheese.
Being careful to cover blender vent with towel, blend simmered cauliflower mixture until smooth. Add additional water to adjust consistency of soup if needed. Blend some more.
Transfer blended soup to bowl.
Garnish with golden florets and drizzled brown butter.
For a dairy-free version, omit Parmesan and butter. Consider using ghee, or clarified butter, as milk proteins are largely removed. If you have a dairy sensitivity or allergy, alternative fat of choice may be substituted--olive oil, tallow, etc.5g net carbs per serving.