Start to boil the potatoes. Once the water gets going, it should take about a half hour, depending on the size. When a fork pierces the potatoes without resistance, drain the water, and run the potatoes under cool water to help remove the peel. Set aside.
Meanwhile, place a large pan over medium heat, and add a tablespoon of oil. Next, add the chopped onion and carrots. Stir and sauté until the onion is tender, about 5 minutes.
Scoop in the ground turkey, and spread it out. Allow it to cook for a few minutes to firm up, and then break the ground meat into chunks. Mix frequently until the meat is done, about 7 to 10 minutes.
Preheat the oven to 350°F.
Now make the sauce. Add the tomato paste, Worcestershire sauce, flour, and stock to the pan. Mix well to help the flour dissolve, and cook until the sauce thickens like gravy, about 5 minutes. Add defrosted peas during the last minute.
While the sauce thickens, make the mashed potatoes. Add the peeled potatoes to a large bowl, pour 3/4 cup milk over the top. Season with salt and pepper to taste. Mash it all together. (Instead, you can also use your favourite mashed potato recipe. Just be sure to watch the added fat.)
Assemble the shepherd’s pie in a deep baking dish. Add the meat and veggie filling, leaving about an inch of clearance on top. Fill that space with the mashed potatoes, and carefully smooth it out with a spatula or the back of a spoon.
Place the uncovered dish in the preheated oven to cook for 30 minutes. It’s a good idea to place a large sheet pan on the rack below in case the filling bubbles out.
Remove the shepherd’s pie from the oven, and allow to cool for a few minutes. This gives the sauce time to thicken. It will be ready to devour after 3–5 minutes.