Step 1In a bowl, whisk together the oil, vinegar, parsley, thyme, pepper flakes, salt and several grinds of black pepper. Stir in the onion, basil, capers and garlic. Spread a little of the marinade on the bottom of a large baking dish. Top with a layer of tomatoes. Spread over a little more marinade, then another layer of tomatoes. Top with the remaining marinade, cover and refrigerate at least 3 hours before serving. The marinated tomatoes will keep up to 2 days.
Step 1Place two slices of bread on a cutting board. Divide half of the cheese slices evenly between the two slices, then top with the sliced tomatoes. Top the tomatoes with the remaining cheese, then cover with the two slices of bread to form the two sandwiches. Heat a large cast-iron skillet or heavy-bottomed sauté pan over medium heat. Add 2 tablespoons butter and swirl in the pan until it is melted and starting to bubble. Place the sandwiches in the pan and grill until the bread is a rich golden brown and the cheese has started to melt. Carefully flip the sandwiches over, add the remaining butter and continue to grill until the other side is browned and toasted. Slice the sandwiches in half and serve immediately.