Preheat oven to 450 degrees.
In a saucepan, over high heat, bring 1 C water to a boil. Add a dash of salt and the cauliflower.
Steam for 2 minutes. Remove from heat and drain. Set aside.
In a large bowl, whisk together half of the sharp cheddar, mascarpone cheese, milk and thyme. Season generously with sea salt and pepper.
Add the steamed cauliflower and toss well to combine.
Transfer to a baking dish and sprinkle with the remaining sharp cheddar.
Cover loosely with foil and bake for 15 minutes. Carefully remove the foil and bake for a further 10 minutes or until golden and bubbly.
Remove from oven, sprinkle with fresh thyme and serve immediately.