Creamy baby turnip soup with smoked trout butter

2 tablespoons butter;

2 leeks (white and tender green parts only), cleaned and sliced (about 11⁄2 cups);

1 medium onion, peeled and diced (about 11⁄2 cups);

2 pounds very small and mild turnips, trimmed and sliced (about 5 cups);

1 medium russet potato, peeled and diced (about 2 cups);

4 cups chicken stock;

11⁄2 teaspoons kosher salt, plus more to taste;

1 cup whole milk or half-and-half;

1⁄4 teaspoon ground nutmeg;

1⁄2 teaspoon ground black pepper, plus more to taste;

Melt the butter in a large pot over medium heat. Add the leeks and onion and a pinch of salt and stir to coat. Cook, stirring occasionally, until softened, about 8 minutes. Add the turnips, potato, stock, and salt. Bring to a boil, reduce the heat, partially cover the pot, and simmer until the vegetables are completely tender, about 25 minutes.
Puree in a blender (working in batches to not fill the blender more than half full) and return it to the pot, or puree the soup directly in the pot with an immersion blender. Stir in the milk and heat through. Season with the nutmeg and pepper, plus more salt if needed.
To serve, ladle the hot soup into serving bowls. Top each serving with 3 tablespoons of Smoked Trout Butter and serve at once.
Make-ahead note: You can make the soup up to 1 day ahead; cool, cover, and refrigerate. Stir well and check the seasoning when you reheat it for serving.