You'll need an aerosol cream whipper to make the emulsion. Use a pasteurized egg if you want to avoid raw eggs.
FOR THE SOUP:
- 3 tbsp. butter
- 1 yellow onion, peeled and sliced
- 1 clove garlic, chopped
- 1 small (1 1⁄2 lb.) head cauliflower, cored and cut into small florets
- 2 1⁄2 cups cups milk
- 3⁄4 cups heavy cream
- Salt and freshly ground black pepper
- 1 pinch freshly grated nutmeg
FOR THE EMULSION:
- 1 large yukon gold potato, peeled
- 3⁄4 cups milk
- 3 Tbsp. butter, cut into pieces
- 1⁄4 cups grated parmigiano-reggiano
- 1 egg yolk
- Freshly ground black pepper
- 2 oz. Crozier Blue cheese or gorgonzola, frozen
For the soup: Melt butter in a large pot over medium-low heat. Add onions and garlic and cook until softened, 10–12 minutes. Add cauliflower, milk, cream, and salt and pepper to taste and simmer, covered, until cauliflower is soft, about 20 minutes. Purée soup in batches in a blender until smooth. Return soup to pot, add nutmeg and adjust seasonings, and keep warm.
For the emulsion: Preheat oven to 250°. Cut potato into eighths. Place potatoes in a small pot, cover with salted water, and bring to a boil over high heat. Reduce heat to medium-low and simmer until tender, 30–35 minutes. Drain potatoes, spread in a single layer on a sheet tray, and let dry in oven for 5 minutes. Put potatoes into a bowl, add 1⁄4 cup of the milk and butter, and beat with an electric mixer until smooth. Press mixture through a fine sieve into a small bowl. Bring remaining milk to a boil in same pot over medium heat, whisk in potatoes, and heat through. Stir in parmigiano-reggiano, transfer to a bowl, and let cool. Add egg yolk to bowl, season to taste with salt and pepper, and mix with an electric mixer until combined. Pass through sieve, then transfer to a cream whipper, following manufacturer's directions when you use it in step 3.
Ladle soup into bowls, grate Crozier Blue over top, and aerate emulsion over cheese. Garnish each bowl with a few capers, if you like.