Preheat the oven to 200°C. Apply baking sheet to your baking pan.
Peel and slice your onion. Apply a spoon of oil into a pan and heat it up. When heated, toss in the onions, sprinkle with a pinch of salt and roast lightly for a couple of minutes, stirring it occasionally. Take care that the onion doesn't turn brown too quickly. When it becomes translucent and slightly brown-ish, lower the heat to a minimum and continue to roast slowly for about 20 minutes, until the onion turns a nice caramelized brown, then remove from heat. Set aside a spoon of onions for decoration.
Give the cauliflower a wash, dry well and separate into florets. Place the florets in a bowl, sprinkle with olive oil, salt and pepper and mix well, then spread the cauliflower over the baking pan and place in oven. Bake for about 20 minutes and stir occasionally, until all the florets are tender and a nice golden brown color.
Place the caramelized onion and the baked cauliflower into a pot, pour over with vegetable stock and bring to boil. Then simmer for a minute or two, to combine the ingredients. Remove from heat and blend into a smooth soup.
Place into bowls and decorate with the remaining caramelized onions. Serve with a good sourdough bread.