1. Rinse the rice under cold water until the water runs clear.
2. In a mixing bowl, combine the rice, chopped onion, pine nuts, currants, parsley, mint, olive oil, salt, and pepper.
3. Lay a grape leaf flat, shiny side down. Place a teaspoon of the rice mixture in the center of the leaf.
4. Fold the sides of the leaf over the filling and roll up tightly.
5. Place the stuffed grape leaves seam-side down in a large pot, packing them tightly together.
6. Add water to the pot until the grape leaves are just covered. Place lemon slices on top.
7. Place a heatproof plate on top of the grape leaves to keep them submerged. Cover the pot and bring to a boil.
8. Reduce heat and simmer for 60 minutes, or until the rice is cooked.
9. Remove from heat and let cool to room temperature before serving.