Preheat oven to 425°
Toss cauliflower and onion pieces with 2 tablespoons olive oil along with salt and pepper to taste. Bake on a rimmed baking sheet for about 20 minutes, or until the cauliflower develops some golden color, onions will begin to brown.
Heat remaining 1 tablespoon olive oil on the sauté function of your instant pot. (if you don't have one see recipe note #1) Add the diced carrot and thyme with a pinch of salt and sauté until soft, reserve about half of the carrots. Once cauliflower is done roasting you may choose to reserve 1-2 small florets per serving of soup you anticipate serving, this is completely optional. (#2)
Add roasted onions and cauliflower to the instant pot along with vegetable broth, ham bone, water and bay leaf. Cover and remember to close your vent. Set and cook for 30 minutes on normal pressure.
Once pressure cooking has finished use the quick release method, remember to be careful and avoid the steam! Remove ham bone, bay leaf and thyme.
Use an immersion blender (or regular blender) and blend contents of the pot until well blended. If you desire a thicker soup use the sauté function and cook the soup to reduce until your desired consistency. Taste for seasoning, add salt and pepper as desired.
Ladle soup into bowls and top with reserved veggies and ham. Parmesan is optional but encouraged!