Heat oil in a medium skillet over medium. Add scallions and garlic and cook, stirring often, until softened, about 2 minutes. Add beans and ¼ cup water and cook, stirring and smashing beans with the side of a wooden spoon until mostly mashed, adding more water 1 Tbsp. at a time if needed, until beans are the consistency of loose mashed potatoes. Stir in 1 Tbsp. lime juice and season with salt and pepper. Toss cilantro with 2 Tbsp. lime juice in a medium bowl and season with salt and pepper. Scoop flesh from avocado into another medium bowl and smash with remaining 1 Tbsp. lime juice with a fork until mostly smooth. Season with salt. Divide bean mixture among bottom halves of rolls and top with cotija, ham, romaine mixture, and chips, if using. Spread avocado mixture on top halves of rolls and close sandwiches.