Preheat the oven to 375 degrees.
In a large heavy skillet, heat the oil over medium heat. Brown the onions and meat, stirring occasionally, and season them with the chili powder, garlic powder and salt. When the meat is cooked and the onions are softened, 6 – 8 minutes, remove the pan from the heat (if using beans rather than meat, add them after the onions are cooked and warm them through).
Meanwhile, spread the chips in the bottom of a large flat baking dish with sides (a metal roasting pan is ideal) or a cast iron skillet.
Top the chips evenly with the meat mixture, then the beans, tomatoes or salsa, peppers or olives (optional) and cheese.
Bake the nachos for 8 – 10 minutes until everything is hot and the cheese is melted, but before the edges of the chips get browned. Serve it immediately, scooping the chips and toppings onto each plate, and topping it with guacamole or avocado, sour cream and extra salsa, if desired.