Preheat your oven to 375 degrees F. Bring a large pot of water to a boil.
Peel the potatoes and cut them into 1-inch cubes. Place the potatoes into the boiling water and cook for 20-30 minutes until fork tender. Drain and return to the pot.
While the potatoes cook, place a large skillet over medium heat and pour in the olive oil. Once hot, add the onion and carrots. Sautee for 5-8 minutes until the carrots are tender and the onions have softened. Transfer the veggies to a bowl.
To the same skillet, add the ground beef, garlic powder, steak seasoning, pepper, salt, red pepper flakes, dried rosemary, and dried thyme. Cook until the beef is browned and cooked through, breaking it up as it cooks.
Once browned, sprinkle the gravy mix over the beef and mix together. Add the beer, water, Worcestershire sauce, peas, carrots, and onions to the skillet. Stir together and then simmer over medium-low heat for 10 minutes until thickened. (see note) Pour the filling into a 9-inch x 13-inch baking dish and set it aside.
Finish making the mashed potatoes by using a hand mixer to whip the potatoes. (Or mash them REALLY well with a potato masher.) Add 4 tablespoons of butter and some of the heavy cream and whip/mix into the potatoes. Add more heavy cream as needed to reach your desired consistency.
Stir in the parmesan cheese and season the potatoes with salt and pepper to taste. Spoon large dollops of potatoes over the beef filling in the baking dish making sure the filling is completely covered. Drizzle 1 tablespoon of melted butter over the potatoes.
Bake for 20-25 minutes of until the top is golden and the filling is bubbling. (If desired, you can broil the top to a deep golden color once the baking time is up.) Remove the shepherd's pie from the oven and let rest for 5-10 minutes before serving.
Notes
You can use a 3-quart casserole dish or a 10 to 12-inch cast iron skillet instead of a baking dish for this recipe. The skillet can be used to cook the filling and then go straight into the oven when you finish adding the topping. (Yay for fewer dishes!)
If the filling sauce is not as thick as you would like once it's finished cooking in the skillet, add some cornstarch slurry to the pan. (1 tablespoon water + 1 tablespoon cornstarch)
I recommend using Guinness, Blue Moon, or a red ale for the beer. Guinness will give a deeper, rich flavor. Blue Moon has a lighter crisp flavor that brightens things up. T the red ale just adds a really nice mellow flavor to the gravy.