Heat olive oil in large frying pan (reserving 2 tablespoons of the oil for later use). Sauté onions with salt until transparent. Add pepper and rice, cook 10 minutes, stirring frequently. Add dill, parsley, scallions, juice of 1 lemon and 1/2 cup of the water. Cook 10 minutes or until liquid is absorbed. Correct seasoning.
Rinse grape leaves very well; drain, separate very carefully. Place shiny side down. Put about 1 tablespoon filling on each leaf near base. Starting at base, fold over, fold in sides, then roll tightly toward tip into cigar shape.
Line bottom of kettle with several of the imperfect leaves (to prevent sticking). Arrange rolls in kettle, side by side, in layers. Add reserved 2 tablespoons olive oil and juice of remaining lemon and 1 cup water. Weight down with heat proof plate. Simmer gently 25 minutes. Add remaining 1 cup water, simmer 25 minutes more. Cover rolls in kettle, then refrigerate. To serve, pile stuffed grape leaves on serving plate; garnish with lemon slices. Serve at room temperature.