Pre-heat oven to 180°C. Heat oil in a large pan and add the onion. Cook for 5 minutes or until translucent. Add mince. Cook for 5 minutes until browned. Add carrot and cook for a few minutes.
Stir in tomato paste and season with salt and pepper. Cook for a few minutes, then add the lentils and stock. Simmer for 20 minutes, then stir in the peas. Simmer for a further 25 minutes.
Line the base of a 30cm x 20cm dish with puff pastry (blind bake if preferred). Spoon beef mix into the pie and cover with remaining puff pastry. Bake for 30 mins or until golden.
Garnish with rosemary sprigs!