01 - Set oven temperature to 400°F.
02 - Heat olive oil in a large skillet over medium heat. Add onion, carrots, and celery and cook, stirring occasionally, until softened, about 5 minutes.
03 - Add minced garlic and cook for 1 minute. Incorporate ground beef, breaking it apart with a spoon, and cook until browned, approximately 6 to 8 minutes.
04 - Stir in tomato paste and cook for 1 minute. Add beef broth, Worcestershire sauce, thyme, rosemary, salt, and pepper. Simmer uncovered for 10 minutes, stirring occasionally.
05 - Mix in frozen peas and cook for an additional 2 minutes. Remove from heat.
06 - Steam or boil cauliflower florets until tender, approximately 10 to 12 minutes. Drain thoroughly.
07 - In a food processor or using a potato masher, blend cauliflower with butter, milk, Parmesan cheese, salt, and white pepper until smooth and creamy.
08 - Transfer beef mixture evenly into a 9x13-inch baking dish. Spread cauliflower mash on top, smoothing with a spatula and optionally creating decorative swirls.
09 - Place in the preheated oven and bake for 20 to 25 minutes until topping is lightly golden and filling is bubbling.
10 - Allow to rest for 5 minutes before serving. Garnish with chopped fresh parsley if desired.