Brown the Beef: In a large skillet, brown 1 lb ground beef (or lamb) over medium heat until fully cooked.
Sauté the Vegetables: Add 1 cup diced onion, 2 cloves minced garlic, and 1 cup diced carrots to the skillet.
Create the Sauce: Stir in 2 tbsp tomato paste, 1 cup Guinness stout, 1 cup beef broth, 1 tbsp Worcestershire sauce, 1 tsp dried thyme, 1 tsp salt, and 1/2 tsp black pepper.
Add Peas: Toss in 1 cup peas (frozen or fresh) and stir to combine.
Boil the Potatoes: In a large pot, boil 2 lb peeled and cubed potatoes in salted water until tender, about 15-20 minutes.
Mash Until Smooth: Drain the potatoes and return them to the pot. Add 1/2 cup unsalted butter, 1/2 cup milk (or cream), 1 tsp salt, and 1/4 tsp black pepper.
Preheat Your Oven: Set your oven to preheat at 400°F (200°C).
Layer the Filling: Spread the beef filling in an even layer in a baking dish. Top with your mashed potatoes.
Bake It Up: Bake in the preheated oven for 25-30 minutes until the top is golden brown and slightly crispy.
Cool Before Serving: Let cool for a few minutes before serving.