Preheat the oven to 300°F. Spray a 2-quart dish (about an 8- or 9-inch square baking dish) with cooking spray and set aside.
Melt the butter in a large skillet over medium-high heat. Add the onion and cook about 5 minutes, until soft and translucent. Add the garlic and cook 1 more minute.
Add the ground beef, rosemary, thyme, salt and pepper. Cook over medium heat until the meat is no longer pink. Drain off the fat.
Stir in the tomato paste, beef broth, Worcestershire sauce, carrots and peas. Cook and stir over low heat just until everything is combined.
Transfer the meat mixture to the prepared baking dish. Spread the mashed potatoes over the top, working from the edges toward the center.
Optional: drag a fork across the potatoes to create ridges, or use the back of a spoon to create swoops through the potatoes. Cover the dish with a piece of foil that has been sprayed with cooking spray.
Bake covered for 25 minutes, or until the filling is hot. Increase the oven temperature to 425°F, remove the foil, and return the dish to the oven for about 10 more minutes, or until the potatoes start to brown.
Let the pie rest for 10 to 15 minutes before garnishing with parsley and serving.