Preheat your oven to 375°F and lightly grease a 9x13-inch casserole dish with butter or cooking spray. This prevents sticking and ensures even browning on the bottom layer.
Heat a large skillet over medium-high heat and brown the ground beef, breaking it into small pieces as it cooks for 5-7 minutes until no pink remains. Drain excess fat, leaving just enough to sauté the aromatics.
Add diced onions and minced garlic to the beef, cooking for 2-3 minutes until fragrant and softened. Stir in tomato paste and Worcestershire sauce, coating the beef mixture thoroughly.
Pour in beef broth and add carrots and peas to the skillet, simmering for 8-10 minutes until vegetables soften slightly. This hearty shepherds pie casserole crowd recipe requires a thick beef filling that won't make watery potatoes.
While the beef mixture simmers, peel and cube potatoes, then boil in salted water for 12-15 minutes until fork-tender. Drain completely and mash with butter and milk until creamy.
Transfer the beef mixture to your prepared casserole dish, spreading evenly across the bottom. Top with mashed potatoes, smoothing the surface with a spatula.
Sprinkle cheddar cheese and fresh parsley over the potato layer, then bake uncovered for 25-30 minutes until the top turns golden brown. Let rest for 5 minutes before serving to ensure clean portions.