Preheat the oven to 400° F. Heat the oil in a medium skillet over medium-high heat. Add the onions and cook until translucent, about 3 minutes.
Add the ground beef and break it apart with a wooden spoon or spatula. Stir in the garlic, dried thyme, salt, and pepper. Stir occasionally until the ground beef is cooked thoroughly, about 6 minutes.
Stir in the tomato paste, worcestershire sauce, frozen peas, and beef broth. Bring to a slow boil the reduced heat and simmer for about 5 minutes. When done, remove from heat and set aside,
Place the frozen potato medallions in a small microwave-safe bowl and prepare per package instructions.
Use a slotted spoon to transfer the beef mixture to a small 12-ounce baking dish. Spread out the beef mixture in an even layer.
Spoon the mashed potatoes over the beef. Place dish on a small baking sheet and bake uncovered for 25 minutes. Broil for 2-3 minutes to brown the top. Let cool for about 5 minutes before serving.
Notes
Brown the Meat Well: Browning the meat helps to develop a deep, savory flavor.Cook the Vegetables: Sauté the vegetables until they're soft. This will help them blend in with the meat and make the pie more flavorful.Use a Thickening Agent: To achieve a rich, gravy-like consistency in your filling, use a thickening agent like flour or cornstarch. Mix it with a bit of water before stirring it into your cooked meat and veggies.Season the Mashed Potatoes: Season your mashed potatoes well with salt, pepper, and a bit of butter or cream. This ensures that the top layer of your pie is just as flavorful as the filling.Layer Properly: Spread the meat mixture evenly on the bottom of your dish, and then layer the mashed potatoes on top. Make sure to spread the potatoes to the edges of the baking dish to prevent the filling from bubbling over the top of the potatoes