In a 4- to 5-quart bowl, mix lemon juice with 1 quart water. Rinse apples, cut in half, and core; thinly slice lengthwise, then cut slices into matchstick-size slivers. As apples are cut, immerse in lemon water.
Drain apples thoroughly in a colander; return to bowl. Add orange juice and oil, and mix gently.
Just before serving, add mint leaves. Mix gently and add salt to taste. Place a lettuce leaf on each of 8 salad plates. Spoon apple salad equally onto lettuce leaves; garnish with mint sprigs.