Step 1

Heat the oil in a large pan over medium heat. Add the onion and garlic and cook, stirring frequently, until soft and translucent, about 5 minutes. Do not brown; reduce the heat if necessary.

Step 2

Add the tomato sauce, chipotle peppers and sauce, sugar, cumin, oregano, and salt. Bring to a gentle boil, then reduce the heat to low and simmer, uncovered, for about 10 minutes. Stir in the chicken and cilantro.

Step 3

Preheat the oven to 425°F and set two racks in the center of the oven.

Step 4

Arrange the tortilla chips in a single layer, overlapping slightly, on two 13 x 18-inch baking sheets. Spread the chicken evenly over the chips, followed by the cheeses.

Step 5

Bake until the cheese is melted, 5 to 8 minutes. Remove the nachos from the oven, then sprinkle with scallions and cilantro. Serve immediately, passing the guacamole, sour cream, and lime wedges separately.