Preheat your oven to 375°F. In a big soup pot or large saucepan, add the vegetable stock. Set the stock aside until your cauliflower is done. Break your cauliflower down leaving large florets.
On a sheet tray, add the cauliflower and drizzle with olive oil. Move the cauliflower around so it is somewhat coated. Sprinkle with 1 teaspoon of the salt and roast for 20 minutes. Flip the cauliflower over and return the tray to the oven and roast for an additional 20 minutes. Turn the oven off. If the cauliflower is tender, place the pieces in the pot with the stock. If not quite tender enough (it should have little to no resistance to a knife or skewer), place the tray into the oven (that is now off) and let it sit for an additional 10 minutes or so. When it is done, add the cauliflower to the stock.
Meanwhile, you can sauté the onion. In a skillet over medium heat, melt the butter. Add the onion and sauté until translucent. Toward the end of cooking, add the pinch of pepper and thyme to the sauté pan. Once the onion is done cooking – you want it to be cooked but not caramelized (without any browning) – add the mixture to the stock.
Add the garlic clove and the coriander to the stock and bring the mixture to a boil over medium-high heat. Reduce the heat to a gentle simmer on low and simmer for 20 minutes, uncovered. Remove the stock from the heat and pour it into a blender and secure the blender lid. VERY CAREFULLY begin to blend the mixture. This will take time, as you want it as smooth as you can, but if you turn the blender on too high it may explode with hot liquid everywhere. So do this slowly until everything is blended well.
Once blended, pour soup back into the pot over low heat and stir in the cream and additional 2 teaspoon of salt. Cook for 10 minutes on a low heat. Add the 1 tablespoon of fresh thyme and the grated asiago cheese and cook for an additional 5 minutes, taste for seasoning, add salt if desired. Serve with the prosciutto, Parmesan tuiles, and a sprinkling of fresh thyme.