Preheat oven to 450 degrees.
Bring 4 quarts water to boil in Dutch oven or stockpot over high heat. Add 1 tablespoon salt and cauliflower florets; cook until fork-tender, about 6 minutes.
Drain cauliflower in colander and rinse under cold running tap water until no longer hot.
Leave cauliflower in colander to drain.
Wipe out stock pot and heat butter over medium heat until melted.
Add onion and cook until softened, about 2 minutes.
Add garlic and cook until fragrant, about 30 seconds; stir in Xanthan Gum or flour until combined, about 1 minute.
Whisk in cream and bring to boil.
Stir in nutmeg, ½ teaspoon salt, ¼ teaspoon pepper, 1 cup cheddar cheese, and 1/2 cup Parmesan cheese until incorporated.
Remove from heat and gently stir in cauliflower until evenly combined.
Transfer mixture into a greased 9x13-inch baking dish.
Top with remaining cheddar cheese, then sprinkle remaining parmesan on top.
Bake until golden brown and sauce is bubbling around edges, about 15 minutes.
Serve immediately.