1) In a pot, melt the butter over medium heat. Add the chopped onion and garlic, and sauté until they become soft and fragrant, about 3–4 minutes. Remove from heat once cooked.
2) Transfer the sautéed onion and garlic to a blender. Add the cilantro (including leaves and stems), pistachios, and chicken or vegetable broth. Blend until the mixture is smooth and creamy. Taste the soup and adjust the seasoning with salt if necessary.
3) Pour the blended soup back into the pot and heat it over medium-high heat. Bring it to a boil, then reduce the heat to a simmer for a few minutes to ensure all the flavors are well combined.
4) Serve the soup hot in bowls. Drizzle with some olive oil and garnish with feta cheese crumbs and additional pistachios for a vibrant presentation. Enjoy!