Feel free to use chicken stock if you prefer. You can use frozen spinach in a pinch. If you don't have truffle salt, any salt will work.If you want to make this in an Instant Pot, you will likely need about the same amount of cook time, or a few minutes less. Note that the nutritional value is calculated without the mushrooms.
Add 2 tablespoons of olive oil, the leeks, cauliflower and 1 teaspoon of salt.
Sauté the leek and cauliflower for a few minutes, stirring occasionally.
Add 4 cups of stock or water to the pot and bring to a boil.
Lower the heat to a simmer and cover the pot.
Simmer for 10 minutes.
Turn off the heat on the pot, take the top off and stir in the spinach. Let it cool while you sauté the mushrooms.
Warm a skillet on a low-medium heat and then add the olive oil, salt, and mushrooms. Stir occasionally, or until the mushrooms are browned and the moisture from them has cooked off.
Blend the soup in batches using a high speed blender, or a food processor. Scoop the blended and lumpy soup between the stock pot and the blender until I have it at the consistency that you prefer. Add more salt to taste.
Serve with mushrooms. Refrigerate or freeze leftovers.