Fresh heirloom tomatoes, rich and oily canned sardines, and briny capers and olives are easy to pack for a picnic and throw together outdoors. Patting the tomatoes dry before assembling prevents the sandwiches from getting soggy.
In a medium bowl, combine the sardines, cherry tomatoes, oregano, olives, and capers. Dress with the olive oil, vinegar, and some salt, and toss to combine.
Using a bread knife, cut off the top of the focaccia, dividing the slab into a thin top and thick bottom. Set the top piece aside. Pat the tomato slices with paper towels if they are very wet. Arrange the slices over the bottom of the bread, overlapping slightly; sprinkle lightly with salt. Distribute the sardine mixture evenly over the tomato slices, then cover with the top slab of focaccia. Cut the sandwich into 6 squares and serve.