Cut your cauliflower into florets and wash. To a pot of boiling salted water, add cauliflower florets and boil for 3-4 minutes.
Drain cauliflower florets and set aside.
Wash and discard dark green leaves of leek. Thinly slice light green and white parts of leek.
In a large skillet, add butter and sliced leek and sauté for 4-5 minutes on medium-low heat.
Add minced garlic and sprinkle flour over leeks and cook for a couple of minutes.
Then add heavy cream, dijon mustard, thyme, salt, and pepper. Stir and let cook on medium-low heat until sauce starts to thicken up.
Grated ½ of parmesan cheese into sauce and combine. Adjust seasoning to cheese sauce.
Add in blanched cauliflower florets and combine with cheese sauce until evenly distributed.
In a casserole dish, add the cauliflower leek mixture and top with the remaining grated parmesan cheese.
Bake at 350℉ for 20-25 minutes or until the gratin is bubbling and the parmesan is golden brown.
Remove from oven and let cool slightly. Serve warm.
