Boil the potatoes. Add chopped potatoes to a pot of salted water. Bring to a boil and cook until fork-tender, about 12–15 minutes. Drain well.

Make the mash. Mash with 2 tablespoons butter and the warm milk or cream. Season with salt and pepper. The mash should be creamy but not runny. Stir in half the cheese if using.

Sauté the aromatics. In a large skillet, heat olive oil over medium. Add onion, carrots, and celery. Cook until softened, 6–8 minutes. Stir in garlic for 30 seconds.

Brown the beef. Add ground beef and cook, breaking it up, until browned and no longer pink. Season with salt and pepper. Drain excess fat if needed.

Build the sauce. Stir in tomato paste and cook 1 minute. Sprinkle flour (or stir in cornstarch slurry). Add Worcestershire and beef broth. Simmer 3–5 minutes until thick and glossy. Taste and adjust seasoning. Stir in peas and a knob of butter for sheen.

Assemble in an air fryer–safe dish. Use a shallow, oven-safe dish that fits your air fryer basket (8-inch round or similar). Spread the beef mixture evenly. Spoon the mashed potatoes over the top, smoothing to the edges to seal. Use a fork to create ridges.

Top and air fry. Sprinkle remaining cheese on top. Air fry at 375°F (190°C) for 12–16 minutes, until the potatoes are golden and the edges are bubbling. Rotate the dish halfway if your air fryer browns unevenly.

Rest and serve. Let the cottage pie sit for 5–10 minutes so the layers set. Garnish with chopped parsley and cracked pepper. Serve with a simple green salad or steamed greens.