Preheat the oven to 350 degrees.
Brown the ground beef in a frying pan. Break up the larger pieces to be sure everything is cooked through.
Pour off any fat from the cooked beef.
Lower the heat to medium.
Add the Worcestershire sauce and mix.
Drain the diced tomatoes and add them to the browned ground beef.
Add the frozen peas to the beef and tomatoes. Cook gently until everything is heated through, about 5 minutes.
Season to taste with salt and pepper.
While the beef and veggies cook, heat the packaged mashed potatoes following the directions.
Lightly spray a glass pie pan with nonstick cooking spray.
Transfer the meat and veggies to the pie plate.
Spread the cooked mashed potatoes over the meat. You can use a spatula for this. Try not to mix the potatoes into the meat. Instead, handle the potatoes like you are frosting a cake.
Sprinkle the top of the potatoes with the shredded cheese.
Place the dish on a rimmed baking sheet. This will keep your oven clean in case of a spill.
Bake until the cheese is melted and the edges of the potatoes are golden, about thirty minutes.
Let the shepherd's pie sit for about ten minutes to make it easier to cut neatly.