Boil the Potatoes: Place cubed Yukon Gold potatoes into a large pot and cover with cold, salted water by 1 inch. Bring to a rapid boil over medium-high heat and cook for 15 to 18 minutes until fork-tender. Drain thoroughly in a colander.

Mash & Season: Return hot potatoes to the warm pot. Mash with melted butter, warm cream, sour cream, garlic powder, salt, pepper, and half of the shredded cheddar. Fold in the egg yolk if using until smooth, rich, and fluffy. Cover to keep warm.

Brown the Meat & Aromatics: Preheat oven to 400°F (200°C). In a large 12-inch oven-safe cast iron skillet or deep Dutch oven over medium-high heat, brown ground beef for 5 to 6 minutes, breaking it into small crumbles. Drain excess grease if needed. Add diced onion, carrots, and minced garlic; sauté for 4 to 5 minutes until vegetables soften.

Build the Savory Gravy: Stir in tomato paste, fresh rosemary, and thyme; cook for 1 minute until fragrant. Sprinkle flour evenly over the meat mixture and stir continuously for 1 to 2 minutes to cook out raw flour taste. Pour in beef bone broth and Worcestershire sauce, scraping up any browned bits stuck to the skillet bottom. Simmer for 4 to 5 minutes until glossy and thick. Fold in sweet peas and corn.

Assemble & Texture the Crust: Spread the savory beef mixture into an even layer in your skillet (or transfer to a greased 9x13-inch baking casserole). Dollop mashed potatoes over the top, gently smoothing with an offset spatula. Use the tines of a fork to create textured ridges and crosshatch swirls across the entire surface (this creates crispy browned edges!). Sprinkle remaining cheddar cheese over the ridges.

Bake to Golden Perfection: Bake at 400°F (200°C) for 20 minutes until the edges are vigorously bubbling and the cheese melts. Switch oven to broil on HIGH for 2 to 3 minutes until potato ridges turn deep golden brown and crispy. Let rest for 10 minutes before scooping into hearty portions.