Preheat oven to 400 F. Grease a casserole dish and set aside.

Bring 4 quarts of water to a rolling boil. Add cauliflower and cook for 3-4 minutes. Remove from water and place into an ice bath to blanch/stop the cooking. Cauliflower will continue to bake in the oven and you don't want it to get mushy.

Melt butter in a medium saucepan over medium heat. Add garlic and cook for 1-2 minutes, or until fragrant. Whisk in flour to butter mixture and cook until smooth and golden in color, about 1-2 minutes. 

Slowly add in the milk and heavy cream whisking as your pour. Add in salt, pepper, rosemary and thyme to cream mixture. Cook for 3-4 minutes, the mixture should be thick enough to coat the back of a wooden spoon. Remove cream mixture from heat and whisk in shredded white cheddar and 1 cup of shredded Grand Cru cheese until smooth.

In the casserole dish, arrange the cauliflower pieces and pancetta. Mix in the cheese mixture evenly. Top with extra cheese and bake for 20-25 minutes, or until cauliflower is tender and the top is golden brown. Top with green onions and enjoy!