Preheat the oven to 425°F.

Spray a 9-inch pie plate with nonstick cooking spray. Set aside.

Peel the potatoes and dice into 1/2-inch cubes.

Place the potatoes in a large pot filled with cold water. Set the pot over high heat and bring to a boil. Boil the potatoes until they can be easily pierced with a fork, 15-18 minutes.

Once tender, drain the potatoes in a large colander and return them to the pot. Add the butter, sour cream, salt, and black pepper. Mash the potatoes until smooth. Add milk and stir gently to combine.

Thinly slice the mushrooms.

Finely dice the onion.

Finely dice the carrot

Finely dice the celery.

In an extra-large skillet over medium-high heat, melt the butter. Add mushrooms and salt. Sauté, stirring frequently, until the mushrooms have released their juices and are nicely browned, 5-6 minutes.

Transfer the mushrooms to a bowl and set aside.

Add the olive oil to the pan. When oil shimmers, add the onions, carrots, and celery. Sauté over medium-high heat, stirring frequently, until they are soft and translucent, 7-8 minutes.

Add the lean ground beef, thyme, nutmeg, salt and black pepper to the skillet. Sauté over medium-high heat, breaking beef up as it cooks, until browned and cooked through, 7-8 minutes.

Return mushrooms to skillet. Add the Worcestershire sauce, tomato paste, corn, and peas to the pan. Stir until the corn and peas are thawed, about 2 minutes.

Reduce the heat to medium. Sprinkle the flour over the beef mixture. Cook, stirring constantly to incorporate and toast the flour, about 1 minute.

Slowly stream the beef broth into the pan. Cook, stirring frequently, until broth begins to bubble and thicken, about 2 minutes.

Remove the skillet from the heat. Transfer the beef mixture to the prepared pie plate and smooth into an even layer.

Spoon the potatoes over the beef mixture, spreading them all the way to the edge. Use the back of the spoon to make small peaks in the potatoes.

Place the pie plate on a large baking sheet. Bake the pie on middle rack of oven until potatoes are lightly golden and the edges are bubbling, 15-17 minutes.

Check to see that pie is done. Remove from oven or add time as needed.

Allow to rest for 7-10 minutes before serving. Refrigerate leftovers for up to 3 days.