In a 4–5 quart Dutch oven or soup pot, heat olive oil over medium-low heat.
Add the leeks, shallots, celery, garlic, and a pinch of kosher salt. Stir well.
Cover and cook over low heat until softened and fragrant, about 8–10 minutes.
Stir in 1 teaspoon kosher salt and the garam masala. Cook for 30 seconds to bloom the spices.
Add the butternut squash, vegetable broth, and apple cider vinegar.
Bring to a gentle simmer over medium heat and cook uncovered for 15–20 minutes, stirring occasionally, until the squash is very tender.
Remove from heat and let cool slightly for about 5 minutes.
Carefully purée the soup in batches using a blender until silky smooth. (Or use an immersion blender directly in the pot.)
Return soup to the pot if needed. Season with additional salt and black pepper to taste.
Stir in the Greek yogurt and fresh lime juice.
Ladle into bowls and garnish with extra yogurt, lime juice, croutons, or fresh herbs if desired.