Pour 1 tablespoon boiling water over raisins in 1-cup measure. Stir in balsamic vinegar. Cover with plastic wrap and let stand 15 minutes to plump raisins. Add zest and juice.
Combine olives, raisin mixture, oregano and pepper in food processor or blender. Whirl until almost smooth.
Split each focaccia horizontally into 2 equal halves. Spread 1-1/2 tablespoons of the olive mixture over one bottom half of focaccia. Top with 1/3 cup cheese, half the red-pepper strips and basil leaves if using. Replace top. Repeat with other focaccia and remaining ingredients. Divide each focaccia into quarters. Makes 4 servings.