Heat oven to 400°F. Grease a 9x13-inch baking dish.
Bring a large pot of salted water to a boil. Blanch cauliflower 3–4 minutes until crisp-tender.
Drain thoroughly and spread on a towel to steam-dry while you make the sauce.
Melt 3 tbsp butter in a saucepan over medium heat. Stir in garlic and cook 30 seconds.
Whisk in flour and cook 1 minute to form a blond roux.
Slowly whisk in half-and-half. Simmer, whisking, until thickened, about 3–5 minutes.
Season with salt, pepper, Italian seasoning, and nutmeg.
Remove from heat. Stir in Parmesan and cream cheese until smooth and creamy.
Fold cauliflower into the Alfredo sauce. Spread mixture in the prepared dish.
Melt remaining 1 tbsp butter. Toss with breadcrumbs, then sprinkle over the casserole. Top with mozzarella.
Bake 18–22 minutes until bubbling and golden. Broil 1–2 minutes for extra color if desired.
Rest 5 minutes. Shower with parsley and serve warm.