In a large skillet, brown the ground beef over medium-high heat. Drain off the fat. Add olive oil, heat, then add the leeks and chilies and saute until the leeks are tender, about 3 minutes. Stir in undiluted chicken broth, ketchup, vinegar, and cumin, and continue cooking over medium heat until the liquid reduces, and the mixture is very thick and richly caramelized in color, about 8 or 10 minutes.

Note: The topping may be pre-: pared to this point, refrigerated, and reheated before assembling nachos.

Fry the corn tortilla quarters, no more than 5 or 6 at a time, in hot oil until golden brown and crisp; drain on absorbent paper towels·. To assemble for nachos, layer the tortilla chips on a large cookie sheet or pizza pan. Sprinkle on about ½ of the cheese, then spoon the leek mixture on top of the chips and cheese, making sure most of the chips get at least a portion of the mixture. Sprinkle on remainder of shredded cheese, then broil until the cheese is melted and golden. Serve immediately, with sour cream and salsa on the side or spooned over the entire platter.
Yields 4 to 6 servings.