How to Make It
Place potatoes in a pot, cover with cold, salted water, bring to a boil over medium-high heat and cook until potatoes are tender, about 20 minutes. Drain, transfer to a large bowl and mash with milk and sour cream. Season with salt and pepper.
Warm a large skillet over medium heat until hot. Add beef and cook, breaking up with a spoon, until no longer pink, about 5 minutes. Add onion, carrot, celery and mushrooms and sauté for 8 minutes. Add garlic and sauté 2 minutes longer. Sprinkle flour on top and cook, stirring, for 2 minutes. Stir in broth, Worcestershire sauce and lentils, bring to a boil, reduce heat to low and simmer for 10 minutes.
Preheat oven to 375ºF. Mist a 9-by-13-inch baking dish with cooking spray. Stir peas into meat mixture and season with salt and pepper. Spoon meat mixture into baking dish and carefully spread potatoes on top. Bake for 30 minutes. If potatoes are not brown enough, turn broiler on high and broil until potatoes are golden, about 2 minutes, watching carefully to avoid burning. Serve hot.