For the soup: Heat a large Dutch oven over medium heat. Fry the bacon until crisp, 10 to 12 minutes. Remove to a plate and drain all but 2 to 3 tablespoons of the bacon fat from the Dutch oven. Reserve the bacon.
Add the yams, garlic, thyme, onion, oregano, cumin, coriander and then a good pinch of salt and pepper. Cook, stirring, until the onions have started to soften and the yams are coated in the bacon grease, 2 to 3 minutes. Add 5 cups of the broth and bring to a simmer. Cook until the potatoes are soft, about 15 minutes.
For the pistachio crumble: While the soup cooks, crumble the bacon and toss with the pistachios, parsley, oregano and turmeric until well combined. Set aside.
Using an immersion blender, puree the soup until smooth. Thin with the remaining stock as needed.
Serve the soup into bowls and swirl in a spoonful of sour cream to each serving. Top with the crumble and some parsley leaves.