Cut cauliflower into florets and place into a large pot with about 1 inch of water in the bottom. Bring to a boil, then reduce heat and steam until it can be pierced with a fork but isn't mushy, 5 to 10 minutes.
Drain well, then return to the warm pot for a few minutes to draw out more moisture. Drain again.
Meanwhile, add bacon to the pot and cook over medium heat until crisp. Remove with a slotted spoon and drain on a paper towel-lined plate. Remove all but 2 tbsp of bacon grease from the pot and return to heat.
Add garlic and sauté until fragrant, about 30 seconds, then add whipping cream and bring to just a simmer. Add the cheddar and stir until melted.
Remove from heat and quickly whisk in xanthan gum to combine. Season to taste with salt and pepper.
Preheat oven to 350F.
Using your fingers, shred cauliflower into small macaroni-sized pieces and place in the bottom of a casserole dish or a cast iron skillet. Sprinkle with bacon. Pour cheese sauce evenly over the cauliflower and top with additional grated cheese, if desired.
Bake until bubbly and browned on top, 15 to 20 minutes. Sprinkle with green onions and serve.