Method

1. Prep: Preheat the oven to 425F and set a baking sheet aside. Place the cashews in a medium bowl and cover with boiling (or very hot) water. Let soak while the vegetables roast in the oven.
2. Vegetables: Cut the top 1/4” off of the head of garlic to expose most of the cloves. Place on a sheet of aluminum foil, then drizzle with oil and wrap it up to seal it tightly. Add the parsnips, potatoes, and onion to the baking sheet and drizzle with oil, salt, pepper, and rosemary, if using. Use your hands to toss them until evenly coated, then spread them out evenly on the baking sheet. Add the foil-wrapped garlic to the baking sheet as well.
3. Roast: Place the baking sheet on the top rack of the oven and roast for 35 to 40 minutes, until the vegetables are golden and tender.
4. Blend: Use tongs to transfer the roasted vegetables to a blender. Carefully unwrap the garlic and squeeze the tender cloves into the blender. Drain the cashews, then add them to the blender along with the vegetable broth and water. (Note: I used an 8-cup blender, but if yours is smaller you may need to work in batches). Blend until smooth and creamy; add more salt and pepper to taste, if necessary.
5. Serve: transfer the soup to a large pot or large saucepan, then stir in the lemon juice (you can also just mix the lemon juice in the blender, but I find the soup easier to serve from a pot). Divide into serving bowls, top as desired, and serve warm. Leftovers will keep in the refrigerator for up to 5 days, or can be frozen for up to 2 months.