Sauté onion in hot oil in a large skillet or Dutch oven over medium-high heat for 25 minutes or until golden. Stir in salt and pepper.
Unfold 1 piecrust, and roll to press out fold lines. Place on a lightly greased baking sheet; roll to a 13-inch circle.
Spoon onion mixture onto piecrust, leaving a 1-inch border; lightly brush border with water.
Roll remaining piecrust to press out fold lines; cut into 1/2-inch strips. Arrange in a lattice design over filling; fold edges up over lattice, pressing to seal. Place olives in some of lattice openings.
Bake at 375° on lower oven rack for 25 to 30 minutes or until golden brown.