Rinse and drain the lentils, and remove any tiny stones.
Bring the lentils and water to a quick boil, then reduce to a simmer. Cook for 20 minutes, drain and set aside.
Heat the oil over medium heat and add the onions. Cook the onions, stirring frequently, till deeply golden brown. This can take up to 30 minutes so don't rush it - that caramelization is crucial to the flavor of this recipe.
Place the lentils, onions, walnuts and three of the eggs in a food processor and process till smooth, scraping down the bowl as needed. Season with salt and pepper.
Grate the remaining egg on the small hole side of a box grater. Reserve some for garnish and stir the rest into the chopped liver.
Notes
Recipe can be easily doubled.
You can freeze the chopped liver if making in advance.