Place beans in a bowl, cover with water and soak overnight. Drain and remove skins.

Heat oil in a large saucepan, add onions and saute until softened, 5 to 7 minutes. Add the reserved beans, sugar, salt to taste, and water. Stir to blend, then cover pan and simmer for 45 minutes, or until beans are very soft. Add the lemon juice, simmer for 5 more minutes, and taste for seasoning.

Puree bean mixture in a blender or food processor until smooth, then transfer to a bowl and chill until firm. To serve, either garnish the top with fresh dill, or invert puree onto a serving dish and garnish with the dill and lemon slices.