In a heavy 10" to 12" skillet, heat 3 tablespoons of the olive oil over a moderate heat until a light haze forms above it. Add the onions & cook for 5 minutes, stirring frequently, until they are soft & transparent but not brown. Add the rice & stir constantly for 2 to 3 minutes, or until the grains are coated with oil. Do not let them brown. Pour in the water, add the salt & a few grindings of pepper & bring to a boil over high heat.
Reduce the heat to low, cover tightly, & simmer for about 15 minutes, or until the rice is tender & has absorbed all the liquid. In a small skillet, heat 1 tablespoon of the remaining olive oil & in it cook the pine nuts until they are a delicate brown. Add them to the rice, then stir in the currants.
In a large pot, bring 2 quarts of water to a boil over high heat. Drop in the grape leaves and immediately turn off the heat. Let the leaves soak for 1 minute, then drain them in a sieve and plunge them into a bowl or pan of cold water to cool them quickly. Gently separate the leaves and spread them, dull sides up, on paper towels to drain.
Layer the bottom of a heavy 2 to 3 quart casserole with 10 of the leaves. Stuff each of the remaining 30 leaves with about 1 tablespoon of the rice mixture. Stack the stuffed leaves, side by side and seam sides down, in layers in the casserole and sprinkle them with the remaining 2 tablespoons of oil and the cold water. Place the casserole over high heat for 3 minutes, reduce the heat to low and simmer, tightly covered, for 50 minutes. Then uncover and cool to room temperature.
To serve, arrange the stuffed grape leaves attractively on a platter or individual plates and garnish with lemon wedges.