01 - Set oven temperature to 400°F.
02 - Boil a large pot of salted water and cook cauliflower florets for 10 to 12 minutes until tender. Drain thoroughly.
03 - Heat olive oil in a large skillet over medium heat. Add onion, garlic, carrots, and celery, cooking until softened, about 5 to 6 minutes.
04 - Add ground beef to the skillet, breaking apart with a spoon. Cook until fully browned, around 8 minutes. Drain excess fat if necessary.
05 - Incorporate tomato paste and cook for 1 minute. Add beef broth, Worcestershire sauce, thyme, rosemary, salt, and pepper. Let simmer for 10 minutes until slightly thickened. Stir in peas, then remove from heat.
06 - Using a food processor or potato masher, blend cooked cauliflower with butter, milk or cream, salt, pepper, and Parmesan cheese (if using) until smooth and creamy.
07 - Spread beef filling evenly in an 8x8-inch baking dish. Top with the cauliflower mash, smoothing the surface completely.
08 - Bake for 20 to 25 minutes until the topping is golden and filling is bubbling. Optionally broil for 2 to 3 minutes to achieve additional browning.
09 - Allow to rest for 5 minutes before serving.