Pease pudding used to be a staple dish in England, but is quite rare now - it's only really seen in the North East and Yorkshire. Which is strange, because it's delicious, cheap and nutritious.
Make sure you have the right kind of peas - they look like yellow lentils or channa dhal, but they are not the same at all. They are a kind of dried pea. Soak them in water for about an hour before you start cooking. After soaking, drain the peas and put them in a saucepan. Finely chop the onion and add that and the ham hock to the pan. Cover with water - about twice the volume as you have peas. Bring to the boil, cover and simmer for about an hour.
After simmering, the onion and the peas should have disintegrated. Remove the ham hock, and increase the heat on the saucepan. Boil the peas until you have a thick, fairly smooth paste. Check for seasoning and add salt if necessary. Add the butter and stir until melted. Place the pease pudding in the fridge until it sets.