This recipe is adapted from Feast Without Yeast by Bruce Semon.
I usually make it with just cauliflower and potatoes, but sometimes I'll add an onion or some red bell pepper. The original recipe called for adding an optional mild poblano pepper. If you can't tolerate nightshades, try substituting one or two large yucca (cassava) roots for the potatoes.
1 large head of cauliflower
About 5 medium red potatoes
1 tablespoon additive-free salt (kosher salt or unrefined sea salt)
Enough water to cover
Optional add-ins of your choice (onions, leeks, peppers, herbs and spices, ham or other meat, dill, etc.)
Wash cauliflower and cut it into about 1" or smaller pieces. If you want to add something like an onion, cut it up and cook it with or just before the cauliflower.
In a large soup pot, saute the cauliflower over medium-low heat with a tablespoon or two of oil. Cook the cauliflower, stirring frequently, until tender.
Meanwhile, wash and peel potatoes and cut into small cubes. When the cauliflower is tender, add the potatoes and enough water to cover the vegetables by an inch or two. I used 10 cups of water last time I made the soup. Stir in the tablespoon of salt, bring to a boil and then simmer until the potatoes are tender.
Allow the soup to cool enough to safely work with, then use a blender or immersable handheld blender to puree the soup until there are no lumps in it. Add more water if desired to make it as thick or as thin as you like. It will thicken a bit as it cools.
Add black pepper, dill, or whatever seasonings you like and reheat to serve. If you want to put cooked meat (or tofu for a vegan version) in your soup, add it at this point too. I haven't tried it, but I imagine a splash of cream or a topping of cheese would be delicious if you can tolerate dairy.
This soup refrigerates well and is even better the next day.