How to Make It
Process mayonnaise, buttermilk, cilantro, chipotle pepper, adobo sauce, lime juice, sugar, 1/2 teaspoon of the salt, and 1/2 teaspoon of the cumin in a food processor or blender until blended and smooth, about 1 minute. Cover and chill until ready to use.
Stir together beans, rice, scallions, and 1/3 cup of the mayonnaise mixture. Cover and chill until ready to use.
Stir together black pepper and remaining 3/4 teaspoon salt and 1/4 teaspoon cumin in a small bowl. Rub pork tenderloin with olive oil, and sprinkle with salt mixture.
Preheat grill to medium (350°F to 400°F). Place tenderloin on oiled grates. Grill, uncovered, turning occasionally, until a thermometer registers 140°F when inserted in thickest portion of tenderloin, 13 to 15 minutes. Let rest 10 minutes. Thinly slice pork.
Spoon about 1/4 cup bean mixture onto each lettuce leaf, and top each with 2 to 3 slices of pork, 1 tablespoon salsa, and 1 tablespoon cheese. Serve with remaining mayonnaise mixture.