Preheat your oven to 200°C/Fan 180°C/Gas 5. Place the onion, celery and carrot into a food processor and whiz a couple of times until finely chopped.
In a large frying pan add the rapeseed 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 for five minutes.
Stir in the peas and half of the rosemary, season with a little salt and milled pepper then tip into a small, deep oven dish.
Coarsely grate the potato and place into clean tea towel and squeeze the potatoes dry. Season with a little salt and 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.