Step 1: Prepare the Potatoes. Peel and cut your russet potatoes into 1-inch chunks. Place them in a large pot, cover with cold water, and add a generous pinch of salt. Bring to a boil, then reduce heat and simmer until fork-tender, about 15-20 minutes. Drain thoroughly.
Step 2: Make the Mash. Return the drained potatoes to the hot pot. Add 2 tbsp butter, milk, salt, and pepper. Mash until smooth and creamy. For an extra fluffy mash, avoid over-mixing. Set aside.
Step 3: Sauté the Aromatics. In a large, oven-safe skillet or Dutch oven, heat olive oil over medium-high heat. Add chopped onion, carrots, and celery. Sauté for 5-7 minutes until softened and slightly fragrant.
Step 4: Brown the Meat. Add ground beef to the skillet with the vegetables. Break it up with a spoon and cook until fully browned, stirring occasionally. Drain any excess fat.
Step 5: Build the Flavor. Stir in minced garlic and cook for 1 minute until fragrant. Sprinkle in flour and cook for another minute, stirring to coat the meat and vegetables. This will help thicken the sauce.
Step 6: Deglaze and Simmer. Pour in beef broth and Worcestershire sauce. Bring to a simmer, scraping up any browned bits from the bottom of the pan. Stir in tomato paste, dried thyme, and bay leaf. Season with salt and black pepper to taste.
Step 7: Add Peas and Thicken. Reduce heat to low, cover, and simmer for 10-15 minutes, allowing the flavors to meld and the sauce to thicken slightly. Stir in frozen peas during the last 5 minutes of simmering. Remove the bay leaf.
Step 8: Assemble the Pie. Preheat your oven to 375°F (190°C). Pour the meat and vegetable mixture into a 9×13 inch baking dish (or use the oven-safe skillet if it’s large enough). Spread evenly.
Step 9: Top with Potatoes. Carefully spoon the mashed potatoes over the meat mixture, spreading it evenly to cover completely. You can use a fork to create ridges on top, which will crisp up beautifully in the oven.
Step 10: Bake to Perfection. Place the baking dish on a baking sheet (to catch any potential overflows) and bake for 25-30 minutes, or until the filling is bubbling around the edges and the potato topping is golden brown and slightly crispy. If desired, broil for the last 1-2 minutes for extra browning on the potatoes.
Step 11: Rest and Serve. Let the shepherds pie rest for 10-15 minutes before serving. This allows the filling to set and prevents it from falling apart when scooped. Serve hot and enjoy!