Tangy Cauliflower Soup

In a large saucepan, melt 2 tablespoons of the butter in the olive oil. Add the onion and garlic and cook over moderate heat until softened, about 5 minutes. Add the cauliflower and coriander and season with salt and pepper. Cover and cook for 5 minutes, stirring occasionally. Add the stock and bring to a boil. Cover and simmer over moderate heat until the cauliflower is softened, about 15 minutes. Puree the soup with an immersion blender until smooth. Alternatively, puree the soup in a blender. In a medium bowl, whisk the egg with the cornstarch. Whisk in the yogurt. Whisk the mixture into the soup and bring to a boil over moderate heat, whisking constantly until slightly thickened, 2 to 3 minutes. Season with salt and pepper and stir in the chopped cilantro. Meanwhile, in a small skillet, melt the remaining 2 tablespoons of butter. Cook over moderate heat, swirling the pan occasionally, until the butter is fragrant and the milk solids begin to turn brown, about 5 minutes. Ladle the cauliflower soup into shallow bowls and drizzle the brown butter on top. Sprinkle with crushed red pepper and serve immediately.