Place 2 pounds Russet potatoes, peeled and cubed in a large pot of salted cold water. Bring to a boil, then simmer until fork-tender, about 15โ18 minutes.
Preheat your oven to 400ยฐF. Lightly butter a 2-quart baking dish.
In a large skillet over medium-high heat, cook 1 pounds ground beef (80/20) until browned, breaking it up as it cooks, about 6โ8 minutes. Drain excess fat, leaving about 1 tbsp in the pan.
Reduce heat to medium. Add 1 yellow onion, diced, 2 carrots, peeled and diced, and 2 celery stalks, diced to the pan. Cook until softened, about 5โ6 minutes. Add 3 garlic cloves, minced and cook 1 minute more.
Stir in 2 tablespoons tomato paste and cook 2 minutes. Add 1 tablespoons Worcestershire sauce, 1 cups beef broth, 1 teaspoons fresh thyme leaves, 1 teaspoons salt, and 0.5 teaspoons black pepper. Stir to combine and simmer until the liquid reduces and the filling is saucy but not soupy, about 8โ10 minutes. Stir in 0.5 cups frozen peas and remove from heat.
Drain and return the potatoes to the pot. Mash with 4 tablespoons unsalted butter and 0.5 cups whole milk or cream until smooth. Stir in 1 egg yolk and 0.8 teaspoons salt (for mash). Taste and adjust seasoning.
Pour the beef filling into the prepared baking dish and spread evenly. Dollop the mashed potatoes on top, then spread into an even layer. Use a fork to create ridges on the surface โ these will crisp up nicely.
Bake at 400ยฐF for 25โ30 minutes until the potato topping is golden and the filling is bubbling at the edges. Let rest 5 minutes before serving.
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