Steam cauliflower over boiling water for 8-10 minutes or until fork tender.
Place hot cauliflower into large bowl. Mix in cream cheese, Dijon mustard, and blue cheese. Use potato masher to blend well until mixture is smooth and cheeses are melted. Season to taste with salt and freshly ground black pepper.
Spoon mixture into shallow baking dish. Sprinkle with Cheddar cheese. Bake until heated through and bubbly, about 10-12 minutes.