This easy version of Shepard's Pie comes from Cook's Illustrated. It calls for using ground lamb, but I prefer ground round - you can make your choice. I like this version because it is all done on the stovetop. Enjoy!
Put the diced potatoes into a large pot and pour in cold water so it covers the potatoes by one inch. Bring to a boil, then reduce to simmer and cook until tender, about 15 minutes.
While the potatoes simmer, cook the beef or lamb in a large skillet over medium heat, breaking up the meat with a wooden spoon, until almost cooked through and the fat has rendered, about 3 minutes. Drain the meat through a fine mesh strainer, discarding all but 1 tablespoon of fat.
Add reserved tablespoon of meat fat to skillet and return to medium heat until shimmering. Add onion and 1/2 teaspoon salt and cook until onion is softened, about 5 minutes. Stir in flour and tomato paste and cook, stirring constantly, until the flour is incorporated, about 1 minute.
Slowly whish in broth, thyme and Worcestershire, scraping up browned bits and bring to a simmer. Return the meat to skillet, cover, and cook over med-low heat until sauce is nicely thickened and flavorful, 6 to 8 minutes.
Drain potoates, return to the pot and mash smooth using a potato masher. Stir in butter and then half and half. Season with salt and pepper to taste. Cover to keep warm.
Stir peas and carrots into the meat mixutre and simmer until heated through, about 2 minutes. Season with salt and pepper to taste.
Serve stew in individual bowls and top with mashed potatoes.