Heat the olive oil in a large skillet over medium heat. Cook the onion and garlic, stirring frequently, until soft and translucent, about 5 minutes. Reduce the heat if they begin to brown.
Add the tomato sauce, chipotle peppers, adobo sauce, sugar, cumin, oregano, and salt. Bring to a gentle boil, reduce the heat to low, and simmer uncovered for approximately 10 minutes.
Stir in the shredded cooked chicken and optional ¼ cup cilantro until evenly combined and warmed.
Preheat the oven to 425°F (220°C). Position two racks near the center.
Arrange the tortilla chips in single, slightly overlapping layers on two rimmed 13 × 18-inch aluminum half-sheet pans.
Divide the warm chicken mixture evenly between the pans. Distribute the Mexican cheese blend and Pecorino Romano evenly over the chicken.
Bake for 5–8 minutes, until the cheese is completely melted and the chicken is thoroughly hot. The exposed chip edges should remain dry and crisp.
Sprinkle with scallions and the remaining ¼ cup cilantro. Serve immediately with guacamole, sour cream, and lime wedges passed separately.