01 - Set the oven temperature to 400°F (200°C) to preheat.
02 - Boil a large pot of salted water and cook the cauliflower florets for 10 to 12 minutes until very tender. Drain well.
03 - Heat olive oil in a large skillet over medium heat. Add chopped onion, minced garlic, diced carrots, and celery. Cook for 5 to 6 minutes until softened.
04 - Add ground beef to the skillet and cook, breaking it apart with a spoon, until browned and fully cooked, about 8 minutes. Drain excess fat if necessary.
05 - Stir in tomato paste and cook for 1 minute. Add beef broth, Worcestershire sauce, dried thyme, dried rosemary, salt, and pepper. Simmer for 10 minutes until the mixture thickens slightly. Stir in frozen peas and remove from heat.
06 - Using a food processor or potato masher, blend the cooked cauliflower with butter, milk or cream, salt, pepper, and optional Parmesan cheese until smooth and creamy.
07 - Spread the beef filling evenly in an 8x8-inch baking dish. Top completely with the cauliflower mash, smoothing the surface.
08 - Bake in the preheated oven for 20 to 25 minutes until the topping is golden and the filling bubbles. Optionally, broil the top for 2 to 3 minutes to enhance color.
09 - Allow the dish to rest for 5 minutes before serving.