Heat the butter over medium heat in a large pot. Add the onions and saute until softened.
Add the potato and cauliflower and saute for a few minutes, until the veggies start to brown.
Add the vegetable broth and bring to a boil. Reduce heat and simmer until the vegetables are cooked through.
If you have electricity and an immersion blender, puree the soup until smooth. Otherwise, mash the potatoes and cauliflower with a potato masher (it won’t be smooth, but it will be delicious).
Stir in the nutritional yeast and season with salt and pepper.