Whisk the egg, evaporated milk, mustard powder, cayenne, nutmeg, garlic, and salt and pepper to taste in a bowl; set aside.
Bring a large pot of salted water to a boil. Cook the cauliflower until tender and almost falling apart, 8 to 10 minutes. Remove with a slotted spoon or sieve. Add the pasta to the same water and cook until al dente, about 10 minutes. Drain, reserving about 60ml pasta water. Save the pot.
Put the egg mixture and cheeses in the empty pot and cook over medium-low heat, whisking constantly, until the cheese melts and the sauce begins to thicken, about 6 minutes. Remove from the heat. Add the cauliflower and puree with an immersion blender until smooth and light. Stir in some of the reserved pasta water to adjust the creaminess of the sauce as needed. Add the pasta to the sauce and toss, season with salt and pepper. Serve, sprinkled with breadcrumbs if desired.