Preheat oven to 375 degrees. Spray 9" deep dish pie pan with cooking spray.
Place peeled and diced potatoes in medium saucepan and add water to cover by 1". Bring to boil and reduce heat to simmer. Simmer until potatoes are tender and fork pierces easily. Drain potatoes and return to heat to allow any additional water to evaporate (10−20 seconds). Add butter, sour cream, salt and onion powder. Whip potatoes until light and fluffy (if they are too thick, add a small amount of milk or additional sour cream to achieve desired consistency). Cover and set aside.
Add ground beef, onions and carrots to large skillet over medium-high heat. Break ground beef into pieces and continue cooking until no pink remains. Add garlic once beef is browned and cook 1 minute. Sprinkle beef with flour and stir until completely incorporated and cook 1 minute. Stir in beef broth, tomato paste, Worcestershire sauce and rosemary and bring to a boil then immediately reduce heat to low. Simmer 5−6 minutes until thickened and remove from heat. Stir in peas and transfer to prepared pie dish.
Spread mashed potatoes over top of beef mixture, making sure to spread potatoes all the way to the edges to seal filling inside. Brush potatoes lightly with beaten egg and bake 25−30 minutes until tips of potatoes turn golden brown. Cool 10 minutes before serving and then enjoy!
One Last Thing
Shepherd’s pie is customarily made with lamb, but beef is tasty, less expensive and much easier to find. When made with beef it's actually called cottage pie!