On one slice of ciabatta bread, layer half of prosciutto, mozzarella and spinach. Drizzle half of vinaigrette over spinach and top with second slice of bread.
Assemble second sandwich the same way.
Brush outside of bread slices with olive oil. Grill sandwich in a grill pan or on a gas grill. Press sandwich down onto grill using a sauce pan or panini press. Grill until bread is toasted and cheese is melted. Serve hot.
Any rustic Italian bread will do. This sandwich is also great with foccacia bread.