Prepare the cauliflower mash: Steam or boil the cauliflower florets until tender, about 10-12 minutes. Drain well. In a blender or food processor, blend the cauliflower with butter, heavy cream, salt, and pepper until smooth and creamy. Set aside.

Cook the meat filling: In a large skillet, brown the ground lamb or beef over medium heat. Drain excess fat if necessary. Add the chopped onion, garlic, carrots, and celery. Cook until the vegetables are softened, about 5-7 minutes. Stir in the tomato paste, Worcestershire sauce (optional), and beef broth. Let the mixture simmer until it thickens slightly, about 10 minutes. Season with fresh rosemary, salt, and pepper.

Assemble the Shepherd’s Pie: Preheat the oven to 375°F (190°C). Spread the meat mixture evenly in the bottom of a baking dish. Top with the creamy cauliflower mash, smoothing it out over the filling. Bake in the preheated oven for 20 minutes, or until the top is lightly golden.

Serve: Garnish with freshly chopped parsley, and serve hot.