Bring a large pot of water to a rolling boil over high heat. Add the cubed red skin potatoes and cook until fork-tender, about 10-15 minutes. Drain the water completely. Mash the potatoes with ¼ cup heavy cream, 4 tablespoons butter, and salt and pepper to taste until smooth and creamy. Set aside while you prepare the filling.

In a large cast-iron skillet or oven-safe pan, melt butter over medium heat. Add the diced carrots, zucchini, and onion. Cook, stirring occasionally, until the vegetables are tender and beginning to caramelize, approximately 5-8 minutes.

Add the ground beef and minced garlic directly to the pan with the vegetables. Break up the meat with your spoon and cook until the beef is completely browned and no pink remains. Drain any excess grease from the pan.

Pour in 1 ½ cups of beef stock and sprinkle the ½ cup of flour evenly over the meat and vegetable mixture. Stir regularly over medium heat until the sauce begins to thicken into a rich gravy, about 3-5 minutes. Fold in the frozen peas and stir until combined.

Spread the creamy mashed potatoes evenly over the meat and vegetable filling, creating peaks and swirls with your spoon. Dot the top with small pieces of butter to encourage golden browning. Bake at 375°F (190°C) for approximately 30 minutes, or until the filling is bubbling around the edges and the potato topping is beautifully golden brown.