Preheat oven to 375 degrees F. Bring a large pot of water to a boil. Once it starts to boil, season it with kosher salt.
Spray an 8 x 8 baking dish with vegetable oil spray.
Cook the cauliflower in the boiling water until crisp-tender, about 5 minutes.
While the cauliflower is cooking, bring the cream to a simmer in a small saucepan. Once it's simmering, whisk in the cream cheese chunks and mustard until smooth.
Stir in 1 cup of cheese and add the salt, pepper and garlic powder and continue whisking until the cheese melts, about 1 to 2 minutes.
When the cauliflower is ready, drain the cauliflower in a large colander and let sit for a few minutes so all of the water drains. If needed, pat between several layers of paper towels to dry the cauliflower thoroughly. Transfer the cauliflower to the baking dish.
Pour the cheese sauce over the cauliflower and stir well to combine. Top with remaining 1/2 cup of grated cheese.
Bake until browned and bubbly hot, about 15 minutes. If you want the cheese to be nice and crisp, turn on the broiler and watch carefully so it doesn't burn. When it's as golden as you like it, remove from the oven and serve.
Refrigerate any leftovers.