Lamb, black pudding & mustard hotpot

INGREDIENTS

• 2 tbsp groundnut or sunflower oil (or dripping)
• 2 large onions , thinly sliced
• 350g black pudding , thickly sliced
• 8 lamb chops (middle neck cutlets), excess fat trimmed
• 900g potatoes , peeled and very thinly sliced
• 3 carrots , thinly sliced
• 2 tbsp grainy mustard
• 20g pack parsley , finely chopped
• 6 sprigs of thyme , leaves only
• 700ml/1¼ pints hot lamb or beef stock or a mixture of stock and water
• knob of butter , melted