Bring a medium saucepan of lightly salted water to the boil. Peel the potato and cut into large chunks. Finely chop the brown onion and garlic. Grate the celery and carrot.

Cook the potato in the boiling water until easily pierced with a knife, 12-15 minutes. Drain and return to the pan. Add the butter, milk and the salt. Mash until smooth. Stir through the shredded Cheddar cheese until well combined. Cover to keep warm.

While the potato is cooking, heat a drizzle of olive oil in a large frying pan over a high heat. Add the onion, celery, carrot and a generous pinch of salt and cook, stirring, until softened, 4-5 minutes. Transfer the veggies to a bowl. Return the pan to a high heat with another drizzle of olive oil. When the oil is hot, add the beef mince and garlic and cook, breaking up the mince with a spoon, until just browned, 2-3 minutes. TIP: Transferring the veggies out of the pan allows the beef to brown!

Add the tomato paste, the water and beef stock pot. Stir to combine, then return the veggies to the pan. Add the baby spinach leaves and stir through until just wilted, 1 minute. TIP: Add a dash of water at the end if the filling seems too thick!

Preheat the grill to medium-high. Transfer the filling to a baking dish. Spread the mash on top. Grill until lightly golden, 5-7 minutes.

Slice the beef and spinach cottage pie with Cheddar mash. Divide between plates to serve.