Melt the butter in a large pot and add the onion and a generous seasoning of salt. Cook over medium-high heat until the onions are soft and translucent, about 5 minutes. Add the garlic and cook until fragrant, about 1 minute more.

Add the broccoli, cauliflower, baking soda, anchovy paste, fish sauce, and chicken broth and bring to a boil. You want the vegetables almost submerged so add a bit more stock, if needed.

Reduce the heat and simmer for 20 minutes, or until the vegetables are completely tender. Add the spinach, parsley, and lettuce. Stir to wilt.

Using an immersion blender, puree the soup completely (or transfer in batches to a regular blender, only filling it about half full and slowly turning the speed up to blend the soup). Blend in the cheeses and adjust seasonings to taste. Thin with more broth, if desired. Serve with a little garnish of cheese.