Eggplant Torta Milanese

Name: Eggplant Torta Milanese
Ingredients: 1 canned chipotle chile in adobo sauce, chopped, plus 1 Tbsp. adobo sauce; ¼ cup apple cider vinegar; 1 cup all-purpose flour; 3 ½ teaspoons kosher salt, divided; 1 ¾ teaspoons freshly ground black pepper, divided; 3 large eggs, beaten; 3 cups panko; 2 pounds Italian eggplants (about 2), sliced lengthwise into 1/2-in.-thick pieces; ¾ cup vegetable oil, divided; 4 large sandwich rolls (such as brioche, telera, or bolillo), halved and toasted; ½ red onion, thinly sliced (about 1 cup); 1 cup packed mixed fresh herbs (such as cilantro, basil, and mint); 8 ounces Oaxaca or fresh mozzarella cheese, pulled into thin strands or shredded; 2 avocados, thinly sliced