Heat the oil in a cast iron skillet over medium-high heat.
Add the onion and garlic and sauté for 5 minutes, stirring occasionally. Add the carrots, celery, and mushrooms and sauté for 5 more minutes—stirring occasionally until softened.
Add the ground beef and season with salt and pepper. Cook until browned, crumbling it with a wooden spoon as it cooks.
Stir the flour in until evenly combined. Then, continue to sauté for 1 minute more—stirring frequently. Stir in ½ cup stock, and then use a wooden spoon to scrape up any browned bits that are stuck to the bottom of the pan.
Immediately stir in the rest of the stock, tomato paste, Worcestershire sauce, herbs and frozen peas until combined. Continue cooking until the mixture reaches a simmer. Then, reduce heat to medium-low to maintain a low simmer and continue cooking for 5 more minutes—stirring occasionally. Taste and season the mixture with salt and pepper as needed.
Remove from the heat and layer the veggie tots over the top in an even layer.
Bake uncovered at 400°F for about 20 minutes or until the tots are lightly golden and the filling has started to bubble up around the edges.
Remove the baking dish from the oven, sprinkle a few extra herbs on top if you would like, and then dish up your servings while the shepherd’s pie is still nice and warm. You can store the pie in sealed containers in the refrigerator for up to 5 days or freeze for up to 3 months.