Chop the vegetables in sizes that will cook evenly at about the same rate. Measure about a cup of cauliflower florets and set aside.
Sauté the onion in a tablespoon of butter until transluscent. Then add another tablespoon of butter and fry the potatoes for a minute.
Add the third tablespoon of butter and sprinkle the 2 tablespoons of flour over everything. Stir and cook the mixture for about a minute.
Pour in a quart of vegetable broth and wisk until smooth. Add the chopped cauliflower and bay leaf. Bring to a boil then lower heat to a simmer. Cook until cauliflower is tender, about 20 minutes.
Turn off the heat. Remove the bay leaf. Season with salt and pepper.
Add the ½ cup heavy cream and puree the mixture with an immersion blender until smooth. Season the soup with a teaspoon of ground nutmeg.
Place the reserved cauliflower florets and a cup of milk in a microwave-safe dish and cook in the microwave 3-4 minutes until tender.
Pour off the warmed milk into the soup and stir. Ladle the soup into serving bowls and add a few pieces of the poached cauliflower to each bowl for texture.
Garnish with sliced scallions and a pinch of nutmeg.