Bring a large pot of generously salted water to a boil. Add cauliflower and cook until crisp tender, about 1 minute. Drain and set aside.
In a large skillet over medium-high heat, melt butter. When foaming subsides, add onion and season generously with salt and pepper. Saute, stirring occasionally, until onions are translucent, 5 minutes.
Add milk and cream cheese and cook, stirring, until cream cheese is melted. Decrease heat to low and add cheddar, gruyere, dijon, garlic, cayenne, and nutmeg. Cook until cheese is melted and smooth. Season to taste with salt and pepper and adjust with additional cayenne and nutmeg, if desired.
Add cauliflower, fold to coat with cheese sauce, and transfer to broiler. Broil until deeply golden and bubbling, about 2-3 minutes.
Garnish with almonds and chives and serve immediately.