Heat the oil in a large skillet over medium heat. Add the carrots, onions, and garlic and cook, stirring, until softened, about 3 minutes. Using a slotted spoon, remove the vegetables from the pan and set aside.

Add the beef to the hot skillet and brown on all sides, about 5 minutes. Add the flour, oregano, salt, and pepper and stir to coat the beef.

Return the vegetables to the skillet and stir in the tomatoes. Bring to a simmer, reduce heat to medium-low, partially cover the pan, and simmer until the beef is cooked through, about 10 minutes.

Meanwhile, place the potatoes in a large saucepan and pour over enough water to cover. Set pan over high heat, bring to a boil, and boil until the potatoes are fork-tender, 8 to 10 minutes. Drain and return the potatoes to the pan. Add the cheese and sour cream and mash until smooth and well combined. Season to taste with salt and pepper. At this point, the potatoes should be slightly thin and easy to spread over the beef mixture. If they’re not, add more sour cream or low-fat milk.

Transfer the beef mixture to a deep dish pie plate or shallow casserole dish. Spoon the mashed potatoes over the top and, using the back of a spoon, make an even layer.

Let cool for 15 minutes, then cover the dish with plastic wrap and refrigerate up to 3 days or freeze up to 3 months. Thaw completely in the refrigerator or microwave before baking.

Preheat the oven to 200ºC. Place the casserole dish on a baking sheet and bake until top is golden brown and filling is bubbly, 20 to 25 minutes (the baking sheet prevents messy oven cleanups, in case the filling bubbles over). Let rest 5 minutes before serving.