Prosciutto and Fontina Panini with Arugula Pesto

Name: Prosciutto and Fontina Panini with Arugula Pesto
Ingredients: For the Sandwich; ¼ cup cider vinegar; 2 tablespoons sugar; 1/2 teaspoon salt; 1 large shallot, thinly sliced; 1 loaf ciabatta, sliced lengthwise; 1/3 pound prosciutto (about 10 slices); 1/3 pound fontina, thinly sliced; For the pesto; ¼ cup toasted pine nuts; 2 garlic cloves; 2 cups baby arugula; ½ lemon, juiced; ¼ cup olive oil