Mash the potatoes with the butter, milk, and salt. You can also use leftover mashed potatoes.
Brown the ground beef in a large skillet over medium to medium high heat.
When the beef is mostly browned, add in peas, carrots, onion, rosemary, thyme, Worcestershire sauce, and garlic. Stir and combine until beef is completely browned.
In a 9 by 13 inch pan, make a layer of the beef mixture and then make a layer of the potatoes.
Bake in the oven for 35 to 40 minutes and broil on high for 5 minutes to brown the potatoes on top.