1⁄4 cup extra-virgin olive oil
1 tablespoon balsamic vinegar
2 teaspoons minced fresh oregano or 1 teaspoon dried
1 teaspoon minced garlic
1⁄2 teaspoon salt
1⁄4 teaspoon freshly ground black pepper
Eight 1⁄2-inch-thick slices ciabatta or other rustic Italian white bread
4 ounces thinly sliced mozzarella cheese
4 ounces thinly sliced mortadella
Whisk the olive oil, vinegar, oregano, garlic, salt and pepper together in a small bowl to blend.
Arrange the slices of bread on a flat work surface, and using a brush, spread the vinaigrette evenly over one side of each slice. Divide the mozzarella equally among the bread slices. Top 4 of the bread slices with the mortadella, and then place the remaining 4 slices of bread on the top, vinaigrette side down, to form 4 sandwiches.
Heat a large skillet or grill pan over medium heat. Add the sandwiches and cook, pressing them occasionally with a large spatula or the bottom of a small heavy saucepan, until the bread is golden brown and the cheese has melted, about 4 minutes per side. Serve immediately.