Sausage and kumara pie

Ingredients

• 500 gram lamb sausages
• 1 tablespoon olive oil
• 1 small onion, finely chopped
• 3 celery stalks, finely chopped
• 400 gram can diced tomatoes
• 1 cup frozen peas
• 1 1/2 tablespoon worcestershire sauce
• 1 tomato, thickly sliced
• 3 cup kumara, mashed
• 30 gram butter, chopped