Preheat oven to 425 degrees F. Fill a large pot with water and bring to a boil. Cook cauliflower for 2 minutes. Remove and plunge in ice water; drain. Keep water boiling and cook macaroni until al dente; drain and set aside. While pasta cooks, press cloves into onion and let sit for 10 minutes.
In a medium-size pot over medium heat, bring milk and bay leaf to a simmer with clove-studded onion. In a medium heavy saucepan, melt butter over low heat. Mix in flour and cook for 2 minutes. Remove bay leaf and onion, then whisk milk into flour mixture; cook for 10 minutes more or until mixture comes to simmer. Add half the gruyere; pass mixture through a fine sieve. Add chives, red pepper sauce and nutmeg. Add salt, pepper & Worcestershire to taste.
Spread macaroni and cauliflower evenly in a 12x10 inch ovenproof dish, pour sauce over both, and sprinkle with remaining gruyere. Bake for 20 minutes.