Make the Mashed Potatoes: Place cubed potatoes in a large pot of cold, salted water. Bring to a boil and cook for 15-18 minutes until fork-tender. Drain thoroughly, then mash until smooth. Stir in melted butter, warm cream, kosher salt, garlic powder, half of the shredded cheddar, and the egg yolk until silky and fluffy. Set aside.

Brown Meat & Build Fond: In a large deep skillet or Dutch oven, brown the ground meat over medium-high heat for 6-8 minutes, breaking it into small crumbles with a wooden spoon. Drain excess rendered grease, leaving 1 tablespoon in the pan.

Sauté Aromatics: Add diced onions and carrots to the skillet; cook for 4-5 minutes until onions are soft and golden. Stir in minced garlic and tomato paste, cooking for 2 minutes to caramelize the paste.

Roux & Deglaze with Red Wine: Sprinkle flour over the meat and vegetables, stirring for 1 minute to cook the starch. Pour in the red wine while scraping up any browned bits stuck to the pan bottom, simmering for 2 minutes until reduced by half.

Simmer Savory Gravy: Pour in beef bone broth, Worcestershire sauce, fresh rosemary, thyme, salt, and black pepper. Bring to a gentle boil, then reduce heat to medium-low and simmer for 8-10 minutes until thick and glossy. Stir in sweet peas, then remove from heat.

Assemble the Pie: Preheat oven to 400°F (200°C). Transfer the hot meat filling into a 9x13-inch baking dish (or keep in a deep oven-safe skillet). Spoon the mashed potatoes over the meat, spreading gently to seal the edges completely. Use the tines of a fork to create decorative ridges across the top, then sprinkle remaining cheddar over the ridges.

Bake & Broil: Bake for 20 minutes until the edges are bubbling vigorously. Turn on the broiler for the final 3-4 minutes to achieve deep golden-brown, crispy potato ridges. Let rest for 15 minutes before slicing and serving so the layers set cleanly!