Set the Instant Pot on the “sautee” mode. Add the mushrooms, red onions, and celery. Place the lid on top of the pot for about 3-5 minutes to allow the pot to heat up and the natural water from the mushroom to be released. Remove the lid and allow the veggies to continue to cook, stirring occasionally until the majority of the water has cooked out (about an additional 8-10 minutes).
Add all of the spices, the bay leaf, millet, the “whole” cauliflower head and 4 cups of water to the pot. Cover and engage the lid. Be sure the steamer valve is set to the closed position. Turn the Instant Pot to the “Manual” mode for 10 minutes.
Juice a lemon and lime to yield 1 Tbsp of each. Set juices aside. Place the other pound of mushrooms and the chopped “sweet” onion in a non-stick pan. Do not add any water or broth. Place the lid on the pan. Cook on a medium-high heat covered for about 3-5 minutes to allow the natural water to be released from the mushrooms. Remove the lid, stir occasionally. Continue to cook for an additional 5 or so minutes until all of the water has been released and the onions are caramelized. Add both juices to the pan, stirring occasionally, continue to sautee the veggies until all juices have been absorbed. Turn the pan off and set aside.
Once the Instant Pot has naturally released the steam for at least 10 minutes, remove the top. Search for the bay leaf and remove it from the pot. Using an immersion blender blend the soup to your desired creaminess. Dip some out into a soup bowl. Top with the Lemon-Lime Mushrooms and enjoy!
Optional: The soup is amazingly delicious, but the addition of Baby Arugula is a nice peppery addition.
The soup will keep in an airtight glass container for 5-7 days.