Preheat your oven to 375° F. Spray a baking dish with olive oil.
Bring a large pot of salted water to a boil. Cut your cauliflower into small florets. Cook until CRISP TENDER - DO NOT OVER COOK. Remove the florets from the pot and PAT AS DRY AS POSSIBLE with paper towels. Transfer to your baking dish.
In a small pot, bring the heavy cream to a simmer. Whisk in the cream cheese, mustard, seasonings and salt + pepper to taste until incorporated. Add ONE CUP of the shredded cheese and mix to melt. Pour this mixture over your cauliflower and toss gently to coat. Spread them out in an even layer and top with the remaining shredded cheddar.
Bake until heated through - about 15 minutes. Turn on broil for two minutes to get the golden brown top. Sprinkle with chopped fresh chives and serve.