Pre-heat oven to 180oC. 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.