Position rack in center of oven and preheat the oven to 400°F.
In a 9 by 13-inch baking dish, spread half of the tortilla chips in one even layer and top with half of each cheese.
Repeat with another layer of chips and the remaining cheeses.
Bake until the cheese is melted and chips are crispy, about 8 to 10 minutes.
Using oven mitts or pot holders, remove the nachos from the oven, sprinkle with the scallions, and allow to cool slightly, about 2 minutes.
Serve either straight from the baking dish or divide among serving plates and serve with Salsa, Guacamole, and sour cream spooned over the top if desired.
YIELD: 4 to 6 servings