Turn your oven to 200/Gas Mark 5.
Place the onion, celery and carrot into a food processor and wiz a couple of times until finely chopped.
In a large frying pan add the olive oil and cook the vegetables until soft but not browned.
Next add the tomato puree and cook for a further minute before stirring in the plain flour.
Pour in the beef stock and brown sauce and bring to the boil. Once boiling turn the heat down and add the chopped leftover roast beef. And simmer away for 5 minutes. Stir in the peas and half of the rosemary, season with a little salt and milled pepper then tip into a smallish deep sided oven dish.
Coarsely grate the potato and place into clean tea towel and squeeze the potatoes dry. Season with a little salt and milled pepper. Cover the cottage pie filling with the grated potato, sprinkle over the remaining rosemary and bake in the oven for about 20 minutes until the top is crispy.
You can pop the thatched cottage pie under a medium grill to crisp up if you like.
Place the onion, celery and carrot into a food processor and wiz a couple of times until finely chopped. Tip the vegetables into a large microwavable bowl and cook for 5 minutes.
Stir the tomato puree and flour into the vegetables then pour in the beef stock, add the brown sauce, chopped leftover beef, peas and half of the rosemary. Cover and pop back into the microwave for a further 5 minutes.
Now transfer the filling to a microwavable serving dish.
Grate the potato as coarsely as you can then ring out any excess water by ringing in a clean tea towel. Season the potato with a little salt and milled pepper and mix in the last of the chopped rosemary. Scatter over the filling and cook in the microwave for 8-10 minutes.
If you would like a crunchy top I would say to pop under a hot grill just before serving.
400g Rooster potatoes, peeled
400g left over roast beef, roughly chopped
1 large onion
1 celery stick
100g frozen peas
1tsp tomato puree
1tsp plain flour
400ml beef stock made with a stock cube
1/2tbsp brown sauce
3 sprigs fresh rosemary, chopped
1tbsp olive oil