Put the hocks in a large saucepan. Cover with water, add one of the onions, the carrot, celery sticks and leaves, peppercorns, parsley stalks and bay leaves and bring to the boil. Reduce the heat and simmer gently for 3 hours until the meat is falling off the bone.
After 2 hours, preheat the oven to 200°C/180°fan/Gas 6. Put the squash, the remaining onion wedges and red pepper in a roasting tin, mix in the oil and add the rosemary. Roast for 45 minutes, stirring halfway through cooking. Microwave the lentils according to the pack’s instructions (45–55 seconds), then stir into the roasted squash along with the creamed horseradish, chopped parsley and seasoning.
Drain the ham hocks. Leave to rest for 10 minutes, then pull off the skin and fat and take out the bones and discard. Use a knife and fork to break the meat into large pieces. Serve with the squash and lentil mixture.
Slow cooker method. Check that 2 ham hocks will fit in the slow cooker before you begin. Put the hocks in a large pan, cover with water and bring to the boil. Put the hocks in the slow cooker with the vegetables and peppercorns and half-fill with the hot water. Cover and cook on high for 5–6 hours.
Ask your butcher whether you need to soak the ham hocks. If they are salty, soak the hocks in water for 24 hours, changing the water twice, then drain.
Keep the cooking liquid with the vegetables and use as the base for soup (it will set like a firm jelly). Bring to the boil, adding some peas or any leftover squash and lentil mixture. Using a stick blender, partly purée the soup and add small shreds of ham before serving.