Heat the butter or oil to medium heat in a heavy soup pot or Dutch oven.
Add the leeks and cook until softened, 5-6 minutes. Add the garlic, and cook for another minute, then add the flour and nutmeg, along with a generous pinch of salt.
Stir and cook for 2-3 minutes, until flour begins to brown. Add the potatoes and broth and bring to a boil.
Reduce to a simmer, and simmer until potatoes are tender, 10-15 minutes.
Add the milk and transfer to a blender to puree, or use an immersion blender.
Transfer back to the pot and add the sour cream. Season to taste with salt and pepper and turn the heat down to low while you make the fried leeks.
Heat the avocado oil in a skillet to medium high and add the leeks. Stir and fry until browned and crispy and drain on paper towels. Add a pinch of salt.
Ladle the soup into bowls, and top with the fried leeks before serving.