Method

1. Using a small sharp knife, peel and chop the onions, carrots and celery. Peel the garlic and use a garlic rusher or grate on the fine side of a box grater. 
2. Heat a large non-stick frying pan over a medium heat. Add half of the butter and once it is melted, tip in the onion, carrots, celery and garlic, stirring to combine. Cook for about 5 minutes until softened, stirring occasionally with a wooden spoon. 
3. Add the steak mince to the pan and cook for another 3-4 minutes or until browned, stirring occasionally and break up any lumps with a wooden spoon. Stir in the flour and cook for 1 minute. 
4. Then stir in the tomato puree and allow to cook for a further minute before adding the stock and worcestershire sauce. Bring to a simmer and then cook for 10 minutes until thickened, stirring occasionally. Stir in the frozen peas and place the mixture in an ovenproof dish.
5. Preheat the oven to 180°C/ 350°F/ Gas Mark 4. 
6. Peel the potatoes and cut into dice. Steam the potatoes in a saucepan for 10-12 minutes until tender. To check if they are ready, pierce with the tip of a sharp knife and you should feel so resistance. Tip into a bowl and mash with a potato masher until smooth. Season with salt and pepper and beat in the rest of the butter and milk. 
7. Spoon the mash on top of the mince, starting with dollops around the edges and finishing in the middle. Spread around with a table knife until you have a nice even layer and then bake for 30 minutes until bubbling and golden.
8. Steam sugar snap peas for 1-2 minutes until tender and serve along with the shepherd's pie.