Wash the cauliflower and remove the leaves and stems. Remove the core, and then cut up the cauliflower into large slices. Make sure the slices are similar in size so they roast at the same rate in the oven.
In a large bowl, season your sliced cauliflower with a generous drizzle of olive oil, salt and pepper. The salt here will flavor your soup, so don't be shy. We recommend at least 3 tablespoons of salt or 2 to 3 handfuls of salt. (You may need to adjust salt levels depending on the size of your hands.) Make sure every piece of cauliflower looks coated with oil.
In a large, oven safe pan or roasting dish, spread your cauliflower slices so that every piece is touching the pan. Put in the oven. In 15 minutes, check on your cauliflower and give it a stir. Put back in the oven until the cauliflower is golden brown, 5 to 10 minutes.
In a medium pot or saucepan, bring your soup to a simmering boil. Taste and add salt and pepper as needed. Stir. Pour into serving bowls.
Add a tablespoon of creme fraiche or sour cream to soup if desired. Serve immediately.