Peel and chop the onion, garlic and carrot, and chop the celery stick into small cubes.
Heat up the oil in a large pan, add the chopped onion and fry it on a medium heat until golden.
Add the garlic, carrot and celery and fry for 2-3 minutes until they begin to soften.
Add the mince and use a spatula / wooden spoon to break it up until no longer pink.
Continue to cook to the mince, stirring occasionally, until all the liquid has been absorbed and the mince is beginning to brown.
Add the tomato paste, stock, Worcestershire sauce and herbs, give it a good stir, then place a lid on and leave it to cook until the veggies are tender, and the gravy has thickened.
Season with salt and pepper.
To make the mash, peel and cut the potatoes into long chip shapes or chunks.
Add them to a pan and cover with cold water.
Leave them to boil on a medium heat until the potatoes are fork tender.
Drain the water, add the salt and butter, and mash well.
Gradually add the milk and mash until creamy and smooth.
Transfer the beef filling to an oven-safe dish in an even layer, and top with the mash.
Use a fork to create ridges in the potato topping and bake in the preheated oven at 200 degrees Celsius (390 Fahrenheit) for 20-25 minutes until golden and piping hot.
Garnish with chopped parsley if you wish.