Bring a large pot of water to a boil. Add 2 tablespoons of salt and the lemon juice. Add the cauliflower florets and cook, uncovered, for 15 minutes. Drain the cauliflower in a colander and set aside.
Meanwhile, in a skillet over medium heat, melt 2 tablespoons of the butter. Add the leeks and stir to coat. Season with salt and cook until tender, about 8 to 10 minutes. Adjust the heat as needed to prevent the leeks from browning. Transfer the leeks and cauliflower to a blender or food processor and puree with some of the stock.
In a small saucepan over medium heat, melt the remaining 4 tablespoons of butter. Add the almonds and lower the heat, stirring occasionally. Remove the pan from the heat when the butter stops foaming, the crackling sound stops and it turns a butterscotch color, 5 minutes. Drain the butter immediately into a 6-quart soup pot, reserving the almonds on a plate.
Stir the cauliflower-leek puree into the brown butter. Add the remaining vegetable stock and simmer for 5 minutes. Season the soup to taste with salt, pepper and nutmeg. Serve the soup garnished with the almonds and parsley.