Remove the core from the cauliflower and chop it into rough florets. Place the florets onto a large baking sheet.
Chop the potatoes into pieces about half the size of the cauliflower florets and toss them onto the baking sheet as well.
Chop the onions into rough 1-2 inch pieces and toss them onto the baking sheet. It doesn't matter if the onion layers stick to each other.
Sprinkle the rosemary leaves over the vegetables. Liberally salt the vegetables and season with pepper to taste. Add the oil and toss the vegetables until evenly coated with the oil, rosemary, and seasoning.
Place the garlic heads in a piece of foil. Drizzle the tops with olive oil and then wrap the foil tightly around the head. Nestle the garlic foil packet amongst the veg on the baking sheet.
Roast vegetables for about an hour, flipping and tossing them here and there with a spatula/spoon to promote even browning. You should have deep caramelization.
When done, remove the from the oven and set aside the garlic foil packet. Pour the lemon juice over the hot vegetables (I just drop the lemon half into the warm pan when I'm done so that it can release some oil too). Using your spatula, toss the vegetables with the lemon juice, scraping the browned bits off the bottom.
Once the vegetables are cool enough for you to handle, add a portion of them to an upright blender along with some vegetable stock. Remove the roasted garlic from the foil and squeeze all of the roasted cloves into the blender as well. Keep doing this in batches until you've blended everything up.
Transfer the blended soup to a large pot. Heat the large pot over medium heat until it comes to a boil. Add more stock or water to thin the soup if necessary and adjust seasoning to taste (more lemon, salt, pepper etc--I almost always add more of each). Serve hot with optional garnishes.
Equipment
Blender
Recipe Notes
Give the vegetables real space on the baking sheet — crowding is the most common reason this soup falls flat. Use two baking sheets if you need to. You're looking for deep golden brown color on the vegetables; not just a light toast.
With only five core ingredients, the stock is doing a lot of heavy lifting. A well-seasoned homemade stock makes a noticeable difference in this soup.
Creamy roasted cauliflower soup with potatoes, rosemary, roasted garlic, and lemon. Naturally vegan with no nuts or cream required. Five core ingredients, deeply caramelized flavor, and blended with good stock.