Shepherd's pie

Name: Shepherd's pie
Prep time: 15 min
Cook time: 55 min
Total time: 1 h 10 min
Cuisine: European, British
Ingredients: butter, for greasing; 1 carrot (approx. 170 g), cut into pieces; 80 g frozen green peas; 1 brown onion (approx. 180 g), cut into quarters; 30 g olive oil; 230 g button mushrooms, cut into quarters; 400 g lamb mince; 50 g stout beer; 1 tbsp Meat stock paste (see Tips); 1 tbsp tomato paste; 1 tbsp cornflour; 1 tbsp water; 600 g mashed potato (see Tips)