Shepherds pie

Ingredients

• 500g potatoes
• 3 tbsp butter
• 2 tbsp olive oil
• 1 onion, chopped
• 500g minced lamb, or quorn
• 2 carrots, sliced
• 1 celery stick, chopped fine
• 1 tbsp worcester sauce
• 500 ml vegetable stock
• Salt and pepper
• 250g mushrooms, sliced
• 2 tsp cornflower, mixed with a little water