You can roast the cauliflower ahead of time while cooking other things in the oven and save it in the fridge for several days, making the soup when you are ready for it. I have not tried freezing the roasted cauliflower but that might work as well.
To roast the cauliflower:
- Cut a cauliflower into flowerettes, discarding the leaves and the toughest part of the central stem.
- Toss the cauliflower in olive oil, salt and pepper.
- Roast until brown around the edges.
A higher or lower temperature, what else is in the oven, etc., will affect the time needed. This is not rocket science. You are going for slightly toasted but still moist cauliflower. It does smell like cabbage as it cooks so I am not sure I'd roast it while baking an angelfood cake. You wouldn't want the cake to smell like old socks!
To make the soup:
- In a soup pot, sauté minced shallot and garlic in olive oil.
- Add a peeled and cubed potato and the roasted cauliflower.
- Cover with chicken broth.
- Simmer until the potato is cooked.
- Purée until smooth.
- Optionally add a can of fat-free evaporated milk.
- Adjust the seasoning with salt and white pepper.