Rinse and drain peas, then soak in hot water for 20 minutes.
Cook chopped onion, thyme, and bay leaf in 25g butter for 15 minutes.
Add drained peas and 1 litre of water, boiling then simmering for 30-40 minutes.
Boil gammon in water, discard the water, and stud a halved onion with cloves.
Blend peas into a purée, then beat in remaining butter, nutmeg, egg, and seasoning.
Tie the pea mixture inside muslin with string, leaving room for expansion.
Place gammon, pea bundle, vegetables, and spices in a pan covered with water.
Simmer for 1-1.25 hours, then rest gammon and reserve 300ml of stock.
Make a roux with butter and flour, whisk in stock, mustard, and cream.
Slice gammon and serve with pease pudding and mustard sauce.