Preheat the oven. Set it to 400°F (200°C) and grease a 2-quart baking dish.
Cook the cauliflower. Steam or boil florets until very tender, about 10–12 minutes.
Drain well and let steam off excess moisture for 3–4 minutes.
Make the mash. Add cauliflower, butter, cream cheese, cheddar, garlic powder, a splash of heavy cream, salt, and pepper to a food processor. Blend until smooth and fluffy. Adjust seasoning and cream if needed.
Set aside.
Sauté the aromatics. Heat oil in a large skillet over medium heat. Add onion, celery, and carrots (if using). Cook until softened, 5–7 minutes.
Stir in garlic for 30 seconds.
Brown the turkey. Add ground turkey, breaking it up with a spatula. Cook until no longer pink and lightly browned, 6–8 minutes. Season with salt, pepper, thyme, rosemary, smoked paprika, and onion powder.
Build the sauce. Stir in tomato paste and Worcestershire.
Cook 1 minute. Pour in broth and bring to a simmer.
Thicken the filling. Sprinkle xanthan gum lightly over the surface while stirring, or stir in almond flour. Simmer 2–3 minutes until the sauce is glossy and lightly thickened.
Fold in peas/green beans if using. Taste and adjust salt and pepper.
Assemble. Spread the turkey mixture evenly in the baking dish. Spoon the cauliflower mash on top and smooth with a spatula.
If you like, sprinkle a little extra cheddar over the mash.
Bake. Place on the middle rack and bake 20–25 minutes, until the top is set and lightly golden at the edges. For more browning, broil 1–2 minutes—watch closely.
Rest and serve. Let it rest 10 minutes to set. Garnish with chopped parsley and serve warm.