Preheat oven to 375 degrees F. Bring a large pot of water to a boil. Season the water with salt.
Spray the baking dish with vegetable oil spray.
Cook the cauliflower in the boiling water until crisp-tender, about 5 minutes. Drain well and pat between several layers of paper towels to dry. Transfer the cauliflower to the baking dish and set aside.
Bring the cream to a simmer in a small saucepan, and whisk in the cream cheese and mustard until smooth. Stir in 1 cup of the cheese, salt, pepper and garlic and whisk just until the cheese melts, about 1 to 2 minutes. Remove from heat, pour over the cauliflower, and stir to combine. Top with the remaining 1/2 cup cheese and bake until browned and bubbly hot, about 15 minutes. Serve.