Make the Mashed Potato Topping: Place the peeled chunked russet potatoes into a large pot, cover with cold salted water, and bring to a rolling boil. Simmer for 15 to 18 minutes until completely fork-tender. Drain well and return potatoes to the hot pot for 1 minute to cook off excess steam. Mash thoroughly, then fold in melted butter, warm cream, grated parmesan, egg yolk, garlic powder, salt, and pepper until velvety smooth. Set aside.

Brown Beef & Aromatics: Preheat oven to 400°F (200°C). In a large 12-inch oven-safe skillet or Dutch oven, heat olive oil over medium-high heat. Add ground beef and cook for 6 to 8 minutes, breaking into crumbles until browned. Drain excess fat if needed, leaving about 1 tbsp in pan. Add diced onions, carrots, and celery; sauté for 5 minutes until tender. Stir in minced garlic and cook 1 minute until fragrant.

Build the Deep Savory Gravy: Stir in tomato paste and flour, cooking for 2 minutes to coat the meat and eliminate raw starch taste. Pour in the red wine, scraping up flavorful browned fond from the bottom of the skillet, and simmer for 2 minutes until reduced by half. Pour in beef bone broth, Worcestershire sauce, rosemary, thyme, smoked paprika, salt, and black pepper.

Simmer to Thicken: Bring the sauce to a gentle simmer and let cook uncovered for 10 to 12 minutes until thick, rich, and glossy. Remove from heat and gently fold in the frozen sweet peas.

Layer & Fork-Crimp: Spread the savory beef filling into an even layer in the skillet (or transfer to a 9x13-inch baking dish). Spoon the mashed potatoes over the top, gently smoothing with an offset spatula. Use the tines of a fork to crosshatch and crimp decorative ridges across the entire surface (these ridges catch the heat and turn irresistibly crispy!).

Bake to Golden Perfection: Bake at 400°F (200°C) for 25 to 30 minutes, until the filling is bubbling hot around the edges and the potato peaks turn golden-brown. For extra color, broil on high for the final 2 minutes. Let rest for 10 minutes before slicing and serving!