1. Prepare the Mashed Potatoes: Start by placing the peeled and quartered russet potatoes in a large pot and cover them with cold water. Add a generous pinch of salt. Bring to a boil over high heat, then reduce heat to medium and simmer for 15-20 minutes, or until the potatoes are fork-tender. Drain the potatoes well, then return them to the hot pot. Add the milk, butter, garlic powder, and half of the shredded cheddar cheese (3/4 cup). Mash until smooth and creamy. Season with salt and pepper to taste. Set aside.

2. Brown the Ground Beef: While the potatoes are cooking, heat the olive oil in a large oven-safe skillet (like a cast iron skillet) or a Dutch oven over medium-high heat. Add the ground beef and cook, breaking it up with a spoon, until it’s fully browned. Drain off any excess grease.

3. Saute the Aromatics and Vegetables: Add the chopped onion, diced carrots, and diced celery to the skillet with the browned beef. Cook for about 5-7 minutes, stirring occasionally, until the vegetables begin to soften. Stir in the minced garlic and cook for another minute until fragrant.

4. Simmer the Filling: Stir in the frozen peas, frozen corn, undrained diced tomatoes, condensed cream of mushroom soup, beef broth, Worcestershire sauce, dried thyme, and black pepper. Bring the mixture to a gentle simmer, then reduce the heat to low and let it cook for 5-7 minutes, allowing the flavors to meld and the sauce to thicken slightly. Taste and adjust salt as needed.

5. Assemble the Pie: If your skillet is not oven-safe, transfer the beef mixture to a 9×13 inch baking dish. Evenly spread the mashed potato mixture over the top of the beef filling, making sure to cover it completely. Use a fork to create decorative ridges on the potato topping, which will crisp up nicely in the oven.

6. Add the Cheesy Topping and Bake: Sprinkle the remaining 3/4 cup of shredded cheddar cheese evenly over the mashed potato layer. Place the skillet or baking dish into a preheated oven at 375 F (190 C). Bake for 20-25 minutes, or until the filling is bubbly around the edges and the cheesy mashed potato topping is golden brown and slightly crispy.

7. Rest and Serve: Once out of the oven, let the shepherd’s pie rest for 5-10 minutes before serving. This allows the filling to set slightly, making it easier to serve clean portions.