01 - Preheat your oven to 400°F.
02 - In a large skillet over medium heat, heat olive oil. Add onion, carrots, and celery. Sauté for 5–6 minutes until softened.
03 - Add garlic and cook for 1 minute. Stir in ground beef and cook until browned, breaking it up with a spoon, about 7–8 minutes. Drain excess fat if necessary.
04 - Stir in tomato paste, cook for 1 minute, then add beef broth, Worcestershire sauce, thyme, and rosemary. Simmer uncovered for 10–12 minutes, until slightly thickened.
05 - Stir in peas, season with salt and pepper, and remove from heat.
06 - Bring a large pot of salted water to a boil. Add cauliflower florets and cook for 10–12 minutes, until very tender. Drain well.
07 - Transfer cauliflower to a food processor. Add butter, milk, Parmesan, salt, and pepper. Blend until smooth and creamy, scraping down sides as needed.
08 - Spread the beef filling evenly in a 9x13-inch baking dish. Top with the cauliflower mash, smoothing with a spatula.
09 - Bake for 20–25 minutes, until the top is lightly golden and filling is bubbling. For a more golden top, broil for an additional 2–3 minutes.
10 - Let rest for 5–10 minutes before serving.