Follow the instructions here to prepare a half batch of this mashed potatoes recipe. Once the mashed potatoes are ready to go, remove from heat and set aside until ready to use.
Meanwhile, heat the oil in a large sauté pan over medium-high heat. Add the ground beef (or lamb) and cook until browned, crumbling it with a wooden spoon as it cooks. Transfer the cooked beef to a separate plate with a wooden spoon and set aside, reserving any grease in the sauté pan that it has left behind. (Or if there is no leftover grease, add an extra tablespoon of oil to the pan.)
Add the onion and sauté for 5 minutes, stirring occasionally. Add the carrots, celery, mushrooms, garlic and sauté for 5-7 more minutes, stirring occasionally, until softened.
Stir the flour in with the veggie mixture until evenly combined, then continue to sauté for 1 minute more, stirring frequently. Stir in the wine, then use a wooden spoon to scrape up any browned bits that are stuck to the bottom of the pan.
Immediately stir in the stock, tomato paste, Worcestershire sauce, bay leaf, herb sprigs 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. Remove and discard the bay leaves and herb sprigs. Stir in the cooked meat Taste and season the mixture with salt and pepper as needed.
Transfer the filling mixture to a 9 x 13-inch baking dish and spread it out in an even layer. Spoon the mashed potatoes on top and carefully spread them out in an even layer as well.
Bake uncovered at 400°F for about 20 minutes, or until the potatoes are lightly golden and the filling has started to bubble up around the edges. (If you would like the potatoes to be a bit more browned, you can turn on the broiler for 1 or 2 extra minutes, keeping a close eye on the potatoes so that they do not burn.)
Remove the baking dish from the oven, sprinkle a few extra herbs on top if you would like, then dish up your servings while the shepherd’s pie is still nice and warm. Or store the pie in sealed containers in the refrigerator for up to 3 days, or freeze for up to 3 months.