To make the filling, prepare an outdoor grill or stovetop grill pan for cooking. Combine the olive oil, lemon juice, pepper, and salt in a small bowl. Brush one side of the eggplant slices with the mixture. Arrange them, oiled sides down, on a grill rack about 4 to 5 inches from medium-hot coals or on a heated grill pan. Cook for about 4 minutes; brush the tops with the olive oil mixture, then turn. Cook about 4 more minutes, or until the slices are tender and lightly browned. Transfer the grilled eggplant to a plate and allow to cool.
To assemble the sandwiches, spread half of the black olive-tomato mixture on each cut side of the focaccia. Arrange the spinach leaves on the bottom half. Top with a layer of tomato slices and then eggplant slices. Sprinkle with pepper and salt. Close the sandwich and press gently. Use a serrated knife to slice the focaccia into 8 wedges just before serving.
This spread will keep for up to 5 days in a covered container in the refrigerator. The assembled sandwich can be wrapped in plastic and refrigerated for up to 2 hours before serving.