Preheat your oven to 350 degrees Fahrenheit. Heat up the butter in a large skillet, or until the butter stops foaming and sizzling.
Add the ground lamb, onion, garlic, celery. Season with salt and pepper. Cook until the meat has browned, and the vegetables have softened, then add the tomato paste, Worcestershire, red wine, and chicken stock. Sprinkle the xanthan gum across the top then stir in.
Add the cooked riced cauliflower, heavy whipping cream, cheddar cheese, Parmesan, thyme, salt, and pepper to a food processor. Blend until the cauliflower whips into a smooth puree.
Add the meat mixture to a casserole dish, then spread the pureed cauliflower across the top.
Bake the shepherd’s pie for 45 minutes, or until browned across the top. Allow to cool for 10 minutes before cutting and serving.