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. Add the beef mince, garlic and another drizzle of olive oil. Cook, breaking up 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. Return the veggies to the pan. Stir to combine. Add the baby spinach leaves and stir through until just wilted, 1 minute. Stir in a dash of water if the filling is very thick.

Preheat the grill to medium-high. Transfer the mince filling to a baking dish and spread the mash on top. Grill until lightly golden, 5-7 minutes. TIP: Keep an eye on the mash topping as it can burn quickly!

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