Preheat the oven. Set to 400°F (200°C).
Grease a 2-quart baking dish.
Cook the cauliflower. Steam or boil florets until very tender, 10–12 minutes. Drain well and let sit in the hot pot for 2 minutes to release extra moisture.
Make the mash. Add butter, cream cheese, heavy cream, garlic powder, salt, and pepper to the cauliflower. Blend with an immersion blender or food processor until smooth and fluffy.
Stir in Parmesan if using. Taste and adjust seasoning.
Brown the beef. Heat a large skillet over medium-high. Add beef and cook, breaking it up, until browned.
If very lean, add olive oil. Drain excess fat if needed.
Sauté aromatics and veggies. Add onion, garlic, mushrooms, and celery to the skillet. Cook 4–5 minutes until softened.
Stir in green beans and cook 2 more minutes.
Build the sauce. Stir in tomato paste, Worcestershire, thyme, rosemary, and smoked paprika. Cook 1 minute to bloom flavors. Pour in beef broth and simmer 3–4 minutes until slightly reduced.
Season with salt and pepper.
Thicken if desired. For a thicker filling, sprinkle xanthan gum evenly over the surface and stir briskly. It thickens fast, so use a light hand.
Assemble. Spread the beef mixture evenly in the baking dish. Spoon the cauliflower mash on top and smooth to the edges to seal in the steam.
Add a light sprinkle of cheddar if desired.
Bake. Place on the middle rack and bake 18–22 minutes, until the top is lightly golden and the edges bubble.
Rest and serve. Let it stand 10 minutes so it sets. Garnish with parsley and serve warm.