In a large pot set over medium heat, heat the olive oil. Add the leek and cook, stirring, until soft, about 6 minutes. Add the garlic and cook for 1 minute. Add the curry powder, cumin, and turmeric, and cook, stirring, until aromatic, 30 seconds to 1 minute. Add the squash, cauliflower, sweet potato, and carrots and stir to coat with the spices, then add the chicken stock, water, and salt, and stir to combine, scraping down the bottom of the pot to make sure all that flavor gets into the soup.
Increase the heat to high to bring the curried squash soup to a boil, then lower the heat simmer, uncovered, 30 minutes.
While the soup is simmering, prepare the meatballs: in a large bowl, combine the ground lamb with the garlic, spices, salt, and pepper, being careful not to overmix (which would lead to dense meatballs.) Roll the lamb mixture into 1-in (2.5 cm) meatballs. In a large nonstick saute pan set over medium heat, heat the olive oil, then add the meatballs and cook, moving the meatballs around from time to time, until browned on all sides (about 8-10 minutes total.) Depending on the size of saute pan you have, you might need to brown the meatballs in two or more batches. Set the browned meatballs aside.
Use a stick blender to puree the soup right in the pot, or transfer it to a stand blender to puree it in batches. Return the pureed soup to medium heat. Add the meatballs and simmer for 10 minutes to incorporate flavors. Taste and adjust seasoning if needed.
SERVING: Serve the soup hot, garnished with croutons and fresh coriander.
STORAGE: Let the soup cool to room temperature. Refrigerate in airtight containers for up to 3 days, or freeze for up to 1 month.
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