Preheat a heavy stock up over medium low heat with a light drizzle of olive oil. Sauté the leeks (or onion) with a pinch of salt until softened and translucent, about 15 minutes.
Add the garlic, turmeric, potatoes, cashews and vegetable stock. Bring everything to a simmer, stir in the nutritional yeast, cover with a lid and cook about 20 minutes until the potatoes and cashews have softened.
Carefully transfer the soup to a powerful blender and purée until silky smooth. (the Vitamix did an exponentially better job at this than my immersion blender).