Peel and cut potatoes into 1-inch chunks, then place them in a pot of cold salted water and bring to a boil—this gives you a head start on the longest-cooking component. While the potatoes cook, dice your onion into 1/2-inch pieces and carrot into 1/4-inch pieces, keeping them separate. Measure out all your seasonings, sauces, and liquids so everything is ready when you need it. I like to do my prep work first because it prevents any scrambling once the cooking gets going.

Heat the cooking oil in a large pan over medium-high heat until shimmering. Add the diced onion and cook for 4-5 minutes, stirring occasionally, until the onion turns golden brown and begins to caramelize—this develops deep, sweet flavors that form the foundation of your sauce. Add the ground beef and carrot, breaking up the meat with a spoon as it cooks for 3-4 minutes until the beef loses its raw pink color and the vegetables begin to soften. Season with salt, pepper, and chili powder, mixing well to distribute the spices evenly throughout the meat.

Increase heat to high for 1 minute to deepen the meat's color and caramelize the spices slightly. Add the tomato paste, all-purpose flour, and cornstarch, stirring constantly for about 1 minute to combine and lightly cook the flour—this prevents lumps in your filling. Pour in just enough water (about 1/2 cup) to create a thick, saucey consistency, then reduce heat to low, cover the pan, and simmer for 15 minutes. The cornstarch and flour will thicken the sauce while simmering, creating a rich, cohesive filling. I use both flour and cornstarch because the combination gives you a filling that's thick but still tender, not starchy.

While the beef filling simmers, check on your potatoes—they should be fork-tender by now (about 15-20 minutes total). Drain them well and return to the pot with butter and room-temperature milk, then mash until you reach your desired consistency (I prefer a slight texture, not completely smooth). Season with salt and pepper, adding a touch more milk if needed for creaminess. In a separate pot, bring salted water to a boil and cook the elbow macaroni according to package directions until just tender, then drain thoroughly and set aside.

In a saucepan over medium heat, melt 2 tablespoons of butter, then whisk in 2 tablespoons of flour, salt, pepper, and mustard powder, stirring constantly for about 1 minute to cook out the raw flour taste without browning. Slowly pour in the milk and chicken stock while whisking continuously to prevent lumps, then reduce heat to low and stir occasionally for 3-4 minutes until the sauce thickens and coats the back of a spoon. Remove from heat, stir in the shredded cheddar cheese until melted and smooth, then gently fold in the cooked macaroni from Step 4, ensuring every piece is evenly coated.

Preheat your oven to 180°C (350°F) and lightly grease a casserole dish. Spread the beef filling from Step 3 evenly across the bottom, then layer the cheese-coated macaroni mixture from Step 5 over the meat, and finally top with the mashed potatoes from Step 4, spreading them gently to create an even layer. Sprinkle the remaining 2 tablespoons of sharp cheddar cheese over the top, then bake for 20-25 minutes until the top is golden brown and the filling bubbles slightly at the edges. Let rest for 5 minutes before serving so the layers set slightly and are easier to portion.